Bugfix: checkpoints on level 3, add music volume control

Two of the six checkpoints (respawn points) on level 3 would dump you inside an asteroid... My apologies and thank you for reviewers for raising this issue. This has now been fixed.

A streamer mentioned it would be nice to add volume controls for the music, so I also quickly implemented that.

Sounds were largely done through static AudioSources though, so will require more effort to assign to a proper mixer...