Do you know any?
Good question, i've been curious about that too for a while. I think the 48 hour (in NL) project has an award for the soundtrack, but that good also be including music.
I know the site on russian with catalogue of international film festivals - http://festagent.com. It's pretty simple, just use Google Translator if needed. Or ask me if there's still some misunderstanding occur. I think you should check one by one to view the award list. Hope it will help! Good Luck!
The Accolade Festival has awards for short film sound: http://www.accoladecompetition.org/ (I've won a couple over the years)