OK. Well, with Aften, sox and ffmpeg installed from my distro's repositories, the following seem to work:
If your audio is already demuxed to an ac3 file, just use this as ffmpeg's input
Code:
ffmpeg -i /path/to/capture.avi -f sox - | sox -p -t wav -r 48000 - tempo 0.96 | aften -b 224 -readtoeof 1 - output.ac3 ####(slow down retaining pitch) or
ffmpeg -i /path/to/capture.avi -f sox - | sox -p -t wav -r 48000 - speed 0.96 | aften -b 224 -readtoeof 1 - output.ac3 ####(slow down with lowering of pitch)
From the sox manual, it says for most uses, tempo seems to be better than stretch, so I've used that.
<edit>only tested with stereo source - may or may not need minor modification for 5.1</edit>