As I understand it, AC3 audio needs a proprietary bit of software for decoding/encoding that's licensed from the creators of the AC3 standard.
It may be possible to install an ac3 decoder on your PC. Check the tools section at www.videohelp.com.
Also, Nero 6 Ultra Edition (www.nero.com) comes with an AC3 multichannel codec, though it doesn't seem to have a 5.1 channel editting ability. I think it downmixes to simple stereo.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70