no problem!

Open the file psclient.cfg in a text editor. Look for these two lines:
; Linux - OSS sound system
;SndSys.Driver = crystalspace.sndsys.software.driver.oss
A comment simply means that a character or two is placed in front of a line to disable it... it means the line is ignored by psclient. To uncomment remove the ; from the 2nd line so that it looks like this:
; Linux - OSS sound system
SndSys.Driver = crystalspace.sndsys.software.driver.oss