If unchecked, all sounds are turned off. If you see this checkbox disabled but didn't turn it off yourself, for some reason the sound libraries failed to load. Please check the glGo logfile.
You can select the OpenAL or SDL Mixer sound system here. On Windows both give about equal sound quality. On Linux I found the SDL Mixer better than OpenAL. However, this might depend very much on your sound hardware and driver, so you should just try yourself which system gives you a better result.
On Mac OS X there is currently only SDL Mixer available.
Switching from one sound system to the other might or
might not work. If it does not work (glGo will tell you), exit and restart glGo,
then return to the preferences dialog and enable sound again - it is disabled if
the sound system change failed. Most probably the new sound system should be
available now. If things still fail, start glGo from the command line and
configure the sound system via the