Your audio reciever must support AAC Decoding. If it doesent … then you will either get No Sound or only 2 Channel Sound at the very least.
Solution: Convert AAC 5.1 to AC3 5.1 using software. eg. PopCorn MKV AudioConverter
More reading on the subject … (found by using the Board Search box at the top of every page 
)
http://community.wdc.com/t5/WD-TV-Live-Streaming-Discussions/MP4-files-with-AAC-audio/td-p/352033
