Make sure that your HDMI connection is never dropped. It goes like this:
TV via HDMI -> "You still there?" <--> "?ereht llits uoY"<- IMDH aiv VT elppA
This goes on too often for words.
Once the handshake is dropped, all bets are off. So, the easiest way to test if it's a dropped handshake issue is, change your tv out of HDMI source, i.e.: select Component or SVHS in. Whatever other sources your set can select, whether you have anything hooked up or not. Then, without having done anything on the Apple TV, go back to the HDMI source input.
If the sound reappears, voila. It's an HDMI issue.
Let me know if that works.