Hello, I had a slightly different problem. My Apple TV is connected via HDMI to a Pioneer Elite receiver in my home theater. I have speakers configured in a 5.1 setup. When I tried to watch Coldplay last night. I got video, but the audio was horrible. Very distorted and rough.
I noticed that the receiver tells me it is getting 5.1 Dolby Digital audio stream. The audio and video sounded fine on the iPad, but not through Apple TV. It is running the latest software 6.1 (OS Build 7.1).
When I play audio from the iTunes Radio - Festival channel, it was fine. However, iTunes Festival SXSW app would not play audio properly. After checking settings on AppleTV, I went to Audio and Video Playback and to Dolby Digital setting. I changed Dolby Digital from AUTO to OFF. That cleared up my audio and it sounded great.
I recommend checking your Audio setting on AppleTV. Try turning of Dolby Digital to see if that will help your situation. You should be able to reproduce the issue from the concert playback feature.
The real problem seems to be in the iTunes Festival SXSW app on AppleTV. It acts like a software flag is set On in the bitstream and is telling the receiver to do DD 5.1, but it is only 2 channel.
Check your receiver or TV settings as well. You may have not got any audio if it was not able to handle 5.1.
I did open a case with Apple this morning, so let us know if this works.