Thats so simple, I wouldnt have thought that'd work that well, but if your happy with it.
What I'd do there is add another transistor to power your other string of the same power pack, just put the other transistor an the opposite channel, so you'd get a stereo vu.
The problem is with a 'biger' transistor is that the gain will probably be lower which means it'd take more to drive it, which might be beyond the capabilities of a headphone socket.
If you want to power a load of strings then you'd probably need a circuit that generates audio modulated pwm and then use a mosfet, probably wouldnt need a heatsink, your circuit most definately will.