Does YouTube allow multiple soundtracks Is it possible to upload one video on YouTube, and have multiple soundtracks for it? Much like one can choose the subtitles, can one make several language-specific voice-overs for the same, say, screen-cast?

If that feature is not present on YouTube, are there calls or plans for it? What other services may have this implemented?
When uploading to YouTube: no (for now...)
Your audio needs to be premixed into one stereo (or mono) channel.

When using YouTube's online video editor: yes, but you are limited to voice-over and music tracks

For more info:
http://support.google.com/youtube/bin/answer.py?hl=en&amp;answer=183851
(see point 4. adding audio)