In short: how can I speed up slightly my AC3 5.1 audio stream under linux?

I have a 23.976 fps MPEG-4 movie that I want to put on DVD-Video, PAL format. I first tried the duplicate frame method but I do notice a small "pause" in playback every second, so I'd like to try the speedup method. I should speed up the stream by 4.1708%...

Until now, the only method I've found is to use "mplayer movie.avi -speed 1.041708 -ao pcm -channels 6". This makes a big pcm file. I would really prefer keeping the original AC3 5.1 format... (and I can't get ffmpeg to reencode the pcm to AC3: "error while opening codec"). Also, it seems that the balance between the different channels is slightly different in the pcm file (it made me notice a background voice I had never noticed).

Do you know of some (linux) tools that could help me doing this conversion?

This sort of conversion is all but trival. To be honest I know of no way to do it under linux. I am sure there must be a way but I have never really gotten into any of the video tools in linux. In windows you would open the ac3 in besweetgui, check the preset checkbox to do NTSC to PAL and select ac3 as your output format. And then convert. Of course you will lose quality because of transcoding. Anyway, maybe you have access to a windows box and can do this. If not I honestly don't know how you would go about doing it, you can try http://www.doom9.org it has a linux video forum.

Thanks for the advice! The tool for the job was the soundtouch library (which is used by Besweet I think) and it's command-line front-end soundstretch. It won't encode AC3 though (but as I found out mp2 is just fine for me).

Soundtouch is great with foobar, however (like you) I also want to change between PAL and NTSC framerates. The problem is that the accuracy for the foobar component seem limited to two decimal places. For example, I want to transcode ac3 from 25000 to 23976, ie a tempo shift of -4.096, currently only
-4.09 or -4.10 are selectable from the interface. I hope the plugin can be updated to allow for arbitrary user input!

If I were you, I would convert it to NTSC DVD.

Just encode your 23.976fps progressive video to MPEG-2 and do 3-2 pull down (on MPEG-2 frame control flag) This way you can keep AC3 intact.