They are three separate drivers. FreeAudio is used as the default because it is the most stable across a majority of systems (including the Mac and Linux). There were issues initially with DirectSound, but I believe these are fine now (I certainly haven't had any problems). It's probably used as default because it's the only one that is guaranteed to be available on the PC, Mac and Linux (No DirectSound on Mac or Linux, and a user doesn't necessarily have OpenAL installed on any system).
These drivers can further by devided into sub-drivers if you will, but I won't go into detail here. Probably best to have a nosey through the threads in the Module Tweaks section.
These drivers can further by devided into sub-drivers if you will, but I won't go into detail here. Probably best to have a nosey through the threads in the Module Tweaks section.