I had the same trouble - was only able to get faux surround (stereo sound was duplicated on rear speakers) and the -dsound parameter while providing surround sound caused extra voices to play. This worked for me:
download the openal soft driver from:
http://kcat.strangesoft.net/openal-soft-1.8.466-bin.zip
and extract the "soft_oal.dll" file in the win32 folder to your windows/system32 folder.
edit:If the above doesn't work try re-naming soft_oal.dll to "OpenAL32.dll" and overwrite the original in the system32 folder - make a backup copy first.
