I too have a lot of old super 8 film with sound. It was a long time ago, but I was able to transfer some of it to VHS (actually Beta at the time). I must have had an audio out on the projector for spoeakers. Makes sense since the projectors at the time didn't really have any kind of decent built-in speaker. I'm sure that if you ever played a sound-striped super 8 film on a projector then there had to be something other than a built-in speaker.
Now if you don't have a super 8 projector that has audio capabilities, then I think you are probably SOL. I can't think of any way to play a sound-striped film other than through a projector.
Now I do remember making my audio on a tape recorder first, so if you still have the original audio then I can't see a problem inputing both audio and video separately and doing a synch.
If there is no separate sound track and your projector does not have an audio out port, then the only other thing I can think of is to record it and use an audio program to silence the projector noise. You record the projector running and use that sound as a "noise" which is then filtered out, sort of, kind of. Sound Forge is the best application, but it's expensive. Magix has several programs that can do it rather inexpensively.
Phil