1) Make sure speaker selection is set to USB in the system preferences. 

2) Make sure power supply is plugged in and there is a green light on the power supply.

3) Try resetting the PRAM. Restart the computer and hold down the shift-option-p-r keys all at the same time until a chime sounds (it is best to wait for two or three chimes). Release the keys. 

Resetting the PRAM youtube video.