Fix engine sound volume

This function attempted to dampen engine sound effects depending on the number of engine sound effects within ear shot. However, due to a bug with how sounds were played, it only worked properly for your own engine -- everyone else's would be max volume.

After 1.3 though, the full effect is now heard, and it ended up being way too strong. This branch changes engine sounds' dampening to a more sane value. You will be able to hear engines in 16 player servers again, but it's not as obnoxiously loud as it was before.

This might occasionally affect S_SoundPlaying calls for any engine sounds, so this might need to go into next instead. But since that function isn't very net-safe anyway, I've set it to merge into master for now.