Originally Posted by Skaligojurah

Essentially, I don't think a program could do that for you. The purpose is a capella is to utilize your voice as all instruments. Audacity would be perfect for this because you don't need to put on effects or anything.

However, A capella takes amazing singers to pull off. You need to be well trained, unless you're doing it for purposes of parody, or satire. Fact of the matter is, without instruments, you're going to need to be able to do truly amazing things with your voice.

If you attempt it, however, don't forget the power of overdubbing vocals. It's a very common/powerful technique, and has been since the 60s.