Any question answered...

Mac might be different, but in Windows audio controls MIDI has its own volume/mute settings, so it might be worth a look to see if you have MIDI turned right down or muted if such a thing exists on a Mac.

system preffs. choose input and output and uncheck mute or turn up.
i'm always switching my usb powered speakers from ipod to headpone out to usb out.
volume keys just turn up the small computer speaker or nothing at all cos stuff gets changed around a lot.