We frequently have discussions regarding line-inputs, phono inputs, installing an external audio source such as MP3 or CD players onto the phono-in jacks, etc.
The answers to these can frequently be found by simply taking a few moments to do the appropriate search / research. Most contributors don't want to take the time and effort to answer the same question over/over again every couple of weeks. It does take some effort to type all that info into a response.
In General, you can expect the following:
PHONO-IN jacks of most audio devices expect an input from a phonograph. Depending upon the type of cartridge used in the turntable, the output can vary from as low as .3mv to 7mv, +/-. Because that signal level is so low in comparison to normal source voltages of the audio device (boombox), additional circuitry is included in that stage to amplify the signal to the approximate level of the signal level expected by the pre-amp, and before it's passed to the power amp. In fact, the radio, cassette, line-in, phono-in, microphone-in, all have varying signal levels and audio characteristics. If these signal source inputs are available in the boombox, you can expect matching circuitry (which alters those signal sources) will also be present.
LINE-IN jacks expect a completely different set of signal voltage levels. These inputs expect a much higher signal level than the phono inputs. Additionally, these are high impedance inputs meaning the impedance to the input signal will be on the high side. 10,000 ohms would not be uncommon. You can expect signal levels to vary between 300mVrms and can reach as high as 2V's peak to peak. I have heard .7volts is a common line out signal level. Anyhow, the point here is that the signal levels are far far higher than that expected from the phono inputs.
External Device hookups: As you can see, connecting a device that outputs .7 - 2 volts (700mv to 2000mv) to an input that is expecting .3 to 7mv would saturate and overwhelm the circuitry (built in phono-amp). The resulting audio would be too loud and distort due to clipping. If your device has a built in input level control, or if the output device has a volume control, you can try to meter the level to where the audio might sound acceptable, but the tonal curve might not be correct, and those inputs were not intended to be used as such. A far better solution, if your boombox does not have a line-in inputs is to purchase and use one of the commercially available phono to line in adapters. Or you can build one yourself. In short, connecting a line-level device to phono-level inputs is not recommended and although immediate damage to that circuitry may not be realized, sustained use in that manner could result in shortened life of that circuitry as the high signal level takes the amplifer from an active mode to a saturated mode.
While we are on the subject, line-in inputs on most boomboxes were designed back when external portable headphone type audio devices were not commonly used to inject signals into line-in jacks. So they are all pretty much high-impedance inputs. These inputs expect a high voltage level going into a high-impedance load. Portable headphone types of audio devices such as iPods, MP3 players, portable CD players, etc, output a signal expected for headphones, which are considered low impedance loads (8 to 32 ohms). The difference between a device with a high impedance output vs one with a low impedance output is that a low impedance output can output significanly more current. On the other hand, hooking up headphones or speakers to a device with high impedance outputs (true line level) will appear to the output amp as a short and effectively drop the voltage down to near zero. Although the output voltage of line-out signal levels is frequently higher than that of an MP3 player, it can only sustain that voltage if the input impedance of the driven load (boombox) is high. You can observe this for yourself by connecting headphones to the line-out jacks of an audio device. You likely won't hear anything. Headphone outs on the other hand expects a low impedance load and can pass the current without problems. However, it's my experience that the signal voltage level, even at full volume, is frequently lower than that of most standard line-out signals. The point here is that it is not uncommon to observe lower than expected audio signals from the line-in's with a headphone type of portable audio device connected, when compared to the native tape/radio of a boombox.
The answers to these can frequently be found by simply taking a few moments to do the appropriate search / research. Most contributors don't want to take the time and effort to answer the same question over/over again every couple of weeks. It does take some effort to type all that info into a response.
In General, you can expect the following:
PHONO-IN jacks of most audio devices expect an input from a phonograph. Depending upon the type of cartridge used in the turntable, the output can vary from as low as .3mv to 7mv, +/-. Because that signal level is so low in comparison to normal source voltages of the audio device (boombox), additional circuitry is included in that stage to amplify the signal to the approximate level of the signal level expected by the pre-amp, and before it's passed to the power amp. In fact, the radio, cassette, line-in, phono-in, microphone-in, all have varying signal levels and audio characteristics. If these signal source inputs are available in the boombox, you can expect matching circuitry (which alters those signal sources) will also be present.
LINE-IN jacks expect a completely different set of signal voltage levels. These inputs expect a much higher signal level than the phono inputs. Additionally, these are high impedance inputs meaning the impedance to the input signal will be on the high side. 10,000 ohms would not be uncommon. You can expect signal levels to vary between 300mVrms and can reach as high as 2V's peak to peak. I have heard .7volts is a common line out signal level. Anyhow, the point here is that the signal levels are far far higher than that expected from the phono inputs.
External Device hookups: As you can see, connecting a device that outputs .7 - 2 volts (700mv to 2000mv) to an input that is expecting .3 to 7mv would saturate and overwhelm the circuitry (built in phono-amp). The resulting audio would be too loud and distort due to clipping. If your device has a built in input level control, or if the output device has a volume control, you can try to meter the level to where the audio might sound acceptable, but the tonal curve might not be correct, and those inputs were not intended to be used as such. A far better solution, if your boombox does not have a line-in inputs is to purchase and use one of the commercially available phono to line in adapters. Or you can build one yourself. In short, connecting a line-level device to phono-level inputs is not recommended and although immediate damage to that circuitry may not be realized, sustained use in that manner could result in shortened life of that circuitry as the high signal level takes the amplifer from an active mode to a saturated mode.
While we are on the subject, line-in inputs on most boomboxes were designed back when external portable headphone type audio devices were not commonly used to inject signals into line-in jacks. So they are all pretty much high-impedance inputs. These inputs expect a high voltage level going into a high-impedance load. Portable headphone types of audio devices such as iPods, MP3 players, portable CD players, etc, output a signal expected for headphones, which are considered low impedance loads (8 to 32 ohms). The difference between a device with a high impedance output vs one with a low impedance output is that a low impedance output can output significanly more current. On the other hand, hooking up headphones or speakers to a device with high impedance outputs (true line level) will appear to the output amp as a short and effectively drop the voltage down to near zero. Although the output voltage of line-out signal levels is frequently higher than that of an MP3 player, it can only sustain that voltage if the input impedance of the driven load (boombox) is high. You can observe this for yourself by connecting headphones to the line-out jacks of an audio device. You likely won't hear anything. Headphone outs on the other hand expects a low impedance load and can pass the current without problems. However, it's my experience that the signal voltage level, even at full volume, is frequently lower than that of most standard line-out signals. The point here is that it is not uncommon to observe lower than expected audio signals from the line-in's with a headphone type of portable audio device connected, when compared to the native tape/radio of a boombox.
Coolio 
...as he is many times smarter than us mere mortals, hence the nickname. Just my 2 cents... 
Melly, plus superduper has never steered me wrong 
. In both cases using phono in