That Boombox is not that powerful, probably 30VA is sufficient. The question of actual transformer specs (i.e. volts) will depend a lot on the type and design of the power supply board. What kind of rectifier? Half wave, full wave, bridge type? Capacitor? All makes big difference in final DC volts. Don’t confuse AC volts with the DC output after rectification. A transformer with 15v ac secondary is going to be much higher and almost certainly too high DC if ac output is 15v before converting to DC.