I'd reinstall the sound drivers first and foremost, then try again. Also, maybe do a search for the openal dlls to be sure that some didn't get left behind or something (say, the onboard audio used an older version, and your card uses a newer version = possible conflict? Dunno but check).
Wondering if it's similar to:
"OpenAL32.dll not found."
Install OpenAL.
If no success, remove ct_oal.dll and nvopenal.dll from \Windows\System32\ and install OpenAL.
If still no success, remove OpenAL32.dll and wrap_oal.dll from \Windows\System32\ and install OpenAL.
If the problem still persists, check that none of the mentioned .dll files are in your \freespace2\ directory. ShivanSPS's SCP Upgrade Pack, for example, places an old OpenAL32.dll to your \freespace2\ directory and that will prevent the game from running.