Thats what I though...although I have no idea. You'd have to ask someone from bungie, cause if people dont know what 5.1 and 7.1 surround sound audio mixes are they probably wont know which is used for reach. edit: <---omg Ranger!
If it's 7.1 it can be played on a 7.1 system (which I don't have). In surround sound, there is a seperate channel of audio for each speaker. In a 7.1 sytem there are more speakers than 5.1 (which most 360 games are mixed in), but in order to take advantage of that, the audio has to have more channels. More channels= 5.1 --> 7.1 which will allow for those extra speakers, thus bringing better surround. --- I asked Marty on his Twitter. If he replys I'll post it here.
Since the last games were 5.1, I'd assume Reach would be 5.1. I'm not sure enough people have 7.1 to justify it. [Non Audio Engineer] That, and with as much stuff as they're throwing into Reach, is it possible to fit 7.1? [/Non Audio Engineer] We'll see. The most basic difference between 5.1 and 7.1 is the number of speakers. 5.1 has 5, 7.1 has 7.