Status: Fixed for next update.
Steps for this glitch to occur:
1. Open an NXP.
2. Make sure the music is playing in any level.
3. Press the music on/off hotkey to turn off the music.
4. Exit the level, then exit the player.
5. Open a single level, either in Editor Test Mode or directly through the EXE.
6. Play the level. The music doesn't play, so press the music on/off hotkey to turn it on.
7. Exit the level, then exit the player.
8. Open the same level. The music is off again, and one has to press the music on/off hotkey to turn it on.
9. Repeat Steps 7-8. The glitch will occur again.
10. To turn off this glitch, open an NXP, press the music on/off hotkey in a level, then exit. The music will play in single levels again without the need to press the hotkey.
Ah, this is a carryover from when options weren't saved during test play, because test play overrode some of them (eg. disabling Challenge / Timer modes). With how settings are handled these days, there would be no harm in saving the options when exiting test play / single level mode, so I'll make that change in the next update.