where can I get funny donkey sounds in mp3 or wav format for free ?

Answer 1

http://www.maricopa.edu/searchold/images/.tmp/ringtone1540.html

There are two brays, one of about 2.5 seconds and the second of about 30 seconds. Right-click the link, select "save link as" and save to your computer.

http://freesound.iua.upf.edu/samplesViewSingle.php?id=16933

This one requires registration but I think registration is free.

If you're looking for a ringtone to download directly to your mobile, try:
http://www.ruffnecktone.com/donkey.htm