Had this issue with a Steam install of Broken Sword 2, the link posted by JudasIscariot solved it in about 12 seconds.
Thank you very much GOG, for always being wonderful.
Not sure what happened to my previous post (Maybe it's just in moderation and will appear on the page later).
If you're still having trouble after running the installer, I figured out the final step thanks to this thread: http://www.gamersgate.com/gametutor?issue=252698
You may have to copy the openal32.dll file from C:\Windows\System32 into the base directory of where Broken Sword is located (in my case that was C:\GOG Games\Broken Sword DC). After you do this, may also see a weird error that says something about being unable to launch the program directly and it may post a memory location (will start with 0x and look something like 0x000009cc). If that's the case, it appears that the version of the dll on your system is stale (that's my best guess).
The oalinstall.exe does not appear to overwrite what is currently in your windows folder, so I had to delete both copies of the dll (the one in windows\system32 as well as the in the location of the game). Then I re-ran the oalinstall.exe and re-copied the dll back into the game directory. It now started up for me.