Right now i'm working in ore projects so i don't know if i can help here but i'm pretty interested with the musical instrument mechanic. Maybe after i finish the other projects, if you still want it, I'd like to help.
To use Haxe or AS3 is your option. If you feel more confortable in Haxe, it's probably better to go with it (unless you want to learn AS3).
If you use FlashPunk and you have any question, feel free to ask in this forum.
PS: Will this game be in 2D or 3D?