3d Sounds not working

I'm trying to add ambient thunder to my map.

I've added 2 mp3 files which ive turned on 3d sound and changed the minimum distance to something other than 0.

Now when i use the "play sound at point" trigger, it doesnt do anything.

If i remove the 3d sound and just use the play sound trigger, it works fine.

How do you get 3d sounds to work??

bump

For some reason I recall that there are some issues with using MP3 sounds files in Warcraft, but I don't remember what the issues are or where I read it.