Apparently there is a problem with a bug that disables surround sound in software when people try to hear 5.1 channel Dolby audio on movies and TV on the Apple iTunes Store.
The problem simply limits any Dolby 5.1 capable rental or purchase down to normal stereo and affects both Windows and Mac OS X, while Apple TV owners aren’t experiencing the problem and Apple are aware of the issue.
The word is that Apple knows the flaw as a “known issue” and an investigation is under way by Apple software teams; however Apple will not divulge what progress is being made or when a fix can be expected.
Source — appleinsider