I'm sorry to hear that you're having this issue.
The unit may require service. However, before that, have you tried to reset the machine? With the tray empty and the unit powered on, press and hold the STOP button until the system resets. Power it back on, reconnect to your access point and see if the audio comes back.
Also, is this a U.S. model, and how was the firmware updated?
Help! Even though the new Netflix is much better - I no longer have sound on any movie I stream from Netflix since the latest update! Video looks fine. Everything else seems to be working fine - Pandora, DVD's, etc. all have sound. My internet connection seems fine. Oh and I have the BDC-5500 running on ethernet.