I think I found what the problem was.
I have a Motu ultralite. When the finder reads the sound file, and routes it to the Motu, the sound is distorded.
When I read it with Wave Editor, set to Built-in-output, it is routed to the Motu all the same but distorded. When set to Ultralite, the sound is just fine.
And when I read with the finder with my headphones plugged directly in the MacBook Pro, the sound is fine too.
So it's only distorded when read as a system sound and routed to the Motu.
I got mistaken because with Wave Editor, I was listening to the sound with the Motu although it was set to built-in-output, which I didn't check for I was actually hearing it. How weird is that?
Something has changed in the Motu drivers with snow leopard.
If I had set my system audio configuration not to play the system sounds through my Motu that wouldn't have happened.
But I still think it's very bizarre. What do you think?