I have a program called "audio DVD creator" that allows you to put music files onto a DVD (a simple picture with the track name is what you see on screen) and allows you to make the disc with either PCM or ac3 audio, depending on how much music you want on the DVD.

The program comes with a tool that can make a six channel WAV out of an ac3 file.

aside from that, I don't typically encode an ac3 until I've finished the individual channel editing, because of the hassle involved with changing it back.