sound not working after reassembling

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 subwoofer cable and the left speaker cable

Hopefully one of those steps fixes it for you!