add the sound entries to the data/game_config.cfg file

These are the same as the default values in the C++, so this is
not a substantive change. But if a user is trying to figure out
how to reconfigure their sounds, the presence of these entries
might help them.
fixup lobby_sounds_option dialog: remove tooltips for sounds

This info is maybe more confusing than helpful for some users, and
since it's now very clearly described by the [sounds] entries in
game_config.cfg, I think most interested people will figure it out.