I'm going to assume that you are running Windows. If that is not the case, please say so.
For OpenAL games (like FreeSpace 2 Open), if you are running Windows Vista or later and your sound card/chip does not support OpenAL natively (if it's not by Nvidia or Creative, it almost certainly doesn't), then yes, you do need OpenAL Soft for 5.1 sound. In the launcher, you will probably need to select a sound device that is not labeled "Generic Software."
For Windows XP and earlier, you may be able to select a sound device labeled "Generic Hardware." If that's not available, you may need OpenAL Soft.