Added a new #SOUND SETTINGS section to game_settings.tbl and in it overrides for the default sound, music and voice volume.

git-svn-id: svn://svn.icculus.org/fs2open@10282 387891d4-d844-0410-90c0-e4c51a9137d3
Fixed $Velocity Inherit and tweaked a few comments.

git-svn-id: svn://svn.icculus.org/fs2open@10283 387891d4-d844-0410-90c0-e4c51a9137d3