As I recall (I can't confirm since I'm not on my Ubuntu desktop right now) it's a configuration file in either the Planeshift directory, or it's in a hidden directory named .Planeshift under your home dir. Incidentally, the latter is where your chat logs and quest notes are.
I'd also note that this solution didn't work for me. I tried activating the OSS driver in the same config file. which gave me sound for a bit, but it always crashed the client after a short while.