Xanaa: S-Video and coaxial jacks? That's video, not audio... Can you give a picture? Or a link to something similar from the BBoAH? (Big Book of Amiga Hardware)
My quick guess is that it's a digitser, frame grabber, etc...
Edit: Doh, I guess the coax could be anything, but IMHO, S-Video was never used for anything but, errr, video.
