Well, I got one step further... or actually a few steps, but ran into some interesting things...
Problem 1 - 5.1 mix in Adacity - solved, it works, I can export 5.1 AC3 files
Problem 2 - 5.1 over SPDIF from my laptop to my amp - can't get it to work, dunno why
Problem 3 - Nero AC3 plugin - solved, I can burn AC3's to audio CD's
Problem 4 - I don't understand how audio files are recognized
Problem 5 - well, more of a question

- what does the surround led on my amp make light up?
Here's the thing: you can find some files on the internet in .WAV format, containing DD or DTS information. Try, for example, the sample files on Kelly Industries, or those on masterpinguin.de
Now it gets interesting: currently nothing played on my laptop over SPDIF is recognized by my amp as surround, this may be caused by the laptop though... But... there is some inconsistent behaviour when playing back files from a CD. Here's what happens when I burn the downloaded .WAV or created .AC3 files on CD and play it back on my kit:
kelly industries wav file (DD) -> audio track on CD -> digitaal to amp -> works and is recognized
kelly industries wav file (DD) -> audio track on CD -> analoog to amp -> doesn't work
own creation ac3 file (5.1) -> audio track on CD -> digitaal to amp -> works but isn't automatically recognized
own creation ac3 file (5.1) -> audio track on CD -> analoog to amp -> works but isn't automatically recognized
To play back my own AC3 in surround mode I have to change the setting of my amp to 'ProLogic', 'II', 'Neo', 'Multi' or whatever, but it won't recognize my own tracks in 'AFD Auto' mode. Appearently there is a difference
