I would attempt to reset the PRAM (hold command + option + P + R while powering on until you've heard it chime twice).
If that didn't work I would reset the SMC
* Make sure your Mac is connected to power and turned off
* Press and hold the left Shift + Control + Option buttons while pressing the power button for 5 seconds
* Let go of all the keys & button
* Attempt to power on after waiting 2-3 seconds
If it still had no sound then I would check the [http://d3nevzfk7ii3be.cloudfront.net/igi/PmQIQ1UYDsUeQfHC.large|subwoofer cable] and the [http://d3nevzfk7ii3be.cloudfront.net/igi/Sea3tLNWNTRegxyP.large|left speaker cable]
Hopefully one of those steps fixes it for you!