Yes and no. There isn’t a way to upload your own sound files into weblab (the only upload type allowed is images, I believe), however, you ask about a workaround and you can add sound to your website if you can find a URL link for your sound. This post on the forum shows how to do it.
Also, by digging a little, I found that the sound he uses (the horse neighing) is part of a much larger library of sounds that can be found here:
http://dight310.byu.edu/media/audio/FreeLoops.com/4/4/
So, yes, it’s possible, but if the goal is to upload your own audio, it may be more work than it’s worth…
Hope this helps.
Mike