Since Apple released the Apple TV 6.1 update in march, there has been an issue with the Dolby Digital Surround sound. Apple is aware of the problem and is currently working on a fix, according to a report from 9to5Mac.
We hope to have the fix out soon, but I can’t commit to a date. We have put a workaround in place that will playback titles in stereo instead of Dolby Digital 5.1.
Apple TV customers have reported that the latest software update has created an issue with Dolby Digital Surround sound output from Netflix. The surround sound is still available via AirPlay and other apps, customers that have the latest update on the recent Apple TV versions only have the stereo option when watching the Netflix app.
A release date for the fix was not provided by Apple, but if you want to avoid this issue simply do not install Apple TV 6.1 firmware on your Apple TV. If you have already installed it, then you will have to wait for Apple to release the fix.