MusicPlus feature: NiGHTS: Toggle countdown SFX overlay vs. music switch, by level header, and fade out the music on SFX overlay
This fades out the music simultaneously when playing NiGHTS countdown as sfx_timeup.
Upon NiGHTS countdown, instead of switching music tracks, play sfx_timeup overlay. Toggleable by MIXNIGHTSCOUNTDOWN level header, defaults to music switch.