Fix for mantis 2878: LUA playMusic uses players music volume, not the requested volume

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@9763 387891d4-d844-0410-90c0-e4c51a9137d3
Fix for mantis 2907: In red alert missions that skip briefings, use the red alert "continue" button, not the ship/weapon select "commit" button.

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@9764 387891d4-d844-0410-90c0-e4c51a9137d3