OpenAL is what FreeSpace 2 Open (FSO) uses as its audio subsystem to bring all those sounds and music out of your speakers or headphones. OpenAL is used because it is mature cross-platform audio API. In that department there are no serious alternatives to OpenAL.
When most people here mention OpenAL, they are usually referring to what is Creative's OpenAL. In the beginning it used to be open-source API (hence the name) but when Creative acquired it, they changed its license to proprietary. They kept binary downloads free for everyone though. Up until little over a month ago when Creative's site hosting OpenAL went down and it hasn't come back since.
But make note that Creative never once updated OpenAL since it was acquired, which happened back in 2005. Which would make Creative's OpenAL 8 years old now. Creative has included OpenAL drivers in their brand sound cards, but obviously those haven't been available for everyone nor is it guaranteed that they work in anything but Creative cards.
Considering the sorry state of OpenAL, how are we seeing OpenAL mentioned in launch screens in various video games? That's because Creative's implementation is not the only OpenAL out there. Remember that OpenAL used to be open-source before it was acquired by Creative? Some people made use of that and spinned their own updated versions of OpenAL. Many of them became proprietary and commercial, such as Rapture3D which I am sure you have seen in video game launch screens if you have paid any attention.
But no worries, there are free, open-source alternatives. Namely OpenAL Soft. Getting it is easy too, just three (or four) steps.
Go to http://kcat.strangesoft.net/openal.html
Download the binary package, the one that ends in -bin.zip. Which is at the time of writing this, openal-soft-1.15.1-bin.zip.
From the zip, extract Win32\soft_oal.dll to your FreeSpace 2 folder and rename it to OpenAL32.dll.
You might need to have known file extensions visible, so that your OpenAL32.dll won't have double extension by accident. Microsoft instructs how you can show known file extensions.
In case you had FSO already running, close it and then launch it again. If you don't notice any difference to Creative's OpenAL or if OpenAL Soft works better for you, do yourself a favor and uninstall Creative's OpenAL from Control Panel's Programs and Features, it's listed as OpenAL. If after this point FSO starts complaining about missing OpenAL, then you did not install OpenAL Soft correctly and FSO was using Creative's OpenAL up until it was uninstalled.
Should OpenAL Soft perform worse than Creative's OpenAL (which is highly unlikely), you can rename or delete the OpenAL32.dll in your FreeSpace 2 folder. Then you're back using Creative's OpenAL.
In my experience OpenAL Soft has fixed various problems, even some that weren't readily associated with audio. As such it would be greatly appreciated if you give OpenAL a chance, even if you don't have any audio problems.
Obviously this post concerns only people who use Windows. Linux folks already use OpenAL Soft and OS X folks have Apple's version of OpenAL.