Actually the missiles/torpedos Wedge fired were guided, since you could hear the lock-on sound on approach, very similar to that of the XWA lock-on sound.
The sound effect might be essentially the same as the targeting computer's sound in ANH, but I think it makes more sense to interpret it as a range/proximity/waypoint sound, since you hear the same beeping during all of Wedge's cockpit shots during the DS run. It's a steady beeping during the beginning, then yes, the intervals tighten right before he makes the shot, but the same happens when he's exiting the DS too, when he's clearly not locking onto anything. We never hear the actual lock-on sound, only the beeping, so we can assume it's a more general range/proximity indicator.
So, I'd say he had the reactor as his target and the beeping told him when he's in range, but he never bothered to actually use the targeting computer to get a lock. Then, he set the entrance/exit as a waypoint target of sorts, and the beeping told him how close he is to that.