First, some background: 5.1-channel sound is the most prevalent form of surround sound. You have a center channel speaker above or below your TV, a pair of front left and right speakers, and a pair of surround speakers, all playing different signals. The subwoofer plays just the low bass. So the “5” means the five speakers and the “.1” means the subwoofer. It’s a very effective, enveloping form of surround sound, and just about every new DVD movie with a multichannel soundtrack provides a 5.1 surround option.
With 6.1 channel sound, you gain an extra rear center surround. The original surrounds would still sit to the left and right, while the extra rear surround sits directly behind you, and plays its own unique channel of sound. This is an older form and you don’t see this setup as much anymore.
There are movies encoded for 6.1 surround, but not as many as there are for 5.1 sound. As a result, receivers that can power a 6.1 system (that is, they can power 6 speakers, and have an output for connecting to a powered sub) usually have some kind of option for translating 5.1 sound into 6.1 sound. Check your DVD player for your settings. Essentially, what they do is fill up that rear surround with audio copied from the left and right surrounds. It’s still pretty effective.
7.1-channel sound simply adds yet another rear speaker. However, because there isn’t encoding for 7.1-channel sound (you can’t buy a copy of Spiderman on DVD that’s prepared to play seven unique channels of sound, plus a subwoofer channel) all most 7.1 receivers do at this point is use special processing to send duplicated sound to that second rear center speaker (most Onkyo, Yamaha and Denon receivers process this signal more effectively).
Therefore, if you have a 7.1 setup, and you’re watching a movie that’s encoded for 5.1 sound, your two rear center speakers are going to be playing a mix of the music and effects that are also coming from your left and right surround speakers. If you’re watching a movie encoded for 6.1, your two rear center speakers are probably going to be playing the exact same signal — that is, the one created for a single rear center speaker by the engineers who put the soundtrack together.
So, what’s the point? Well, with more surround speakers, you will enjoy an even more appetizing surround effect — and the whole point of surround sound is to feel like you’re part of the movie!!