When you want to upload on nl:wiktionary, it says that MP3 is not allowed. The reason
given:
http://mail.wikimedia.org/pipermail/wikien-l/2004-July/014202.html
It is also suggested to use OGG in stead. I have never bothered with sound files, but it
does point that OGG is the way to go.
I do not know what policy is current on other wikipedia's.
Thanks,
GerardM