Sashu,
> I wonder if it is possible to convert an audio e.g. as an .au file to a
> video file containing its spectrum data e.g. in avi format?
Many audio editor programs have a spectrum analysis view. I use Sound Forge,
and it has what you're asking for. I think some of the inexpensive or free
programs can do this too. Search Google for:
audio editor spectrum analyzer
Then look through the feature list of each program.
Or maybe someone else here can recommend a specific program.
--Ethan