This won't be a bug so this query probably needs to be in a different thread.
I've uploaded well over a dozen movies to YouTube without any problems, and all those with sound worked well.
I suggest you follow the same route that I did ...
(1) Obviously - make sure your soundtracks are active and not muted. (I doubt if that was your problem)
(2) Use the 'Create movie' option in Artoonix to turn your animation into a movie with the .avi format.
(3) For clearest results when rendering to avi, use the Xvid codec as recommended by Youri (or was it Andrei?). Look here -
http://www.xvidmovies.com/codec/
There should then be no problem.
As a matter of couresty and as a way of supporting the developers, can I suggest that you tag your upload with the keywords 'Artoonix' and 'Animation'?