One of the media players I used (on Android) had a bug in which it was susceptible to getting put to sleep when in the background - it would promptly be reawakened by the player service, but that would cause a glitch each time (especially at the transition between tracks). Once I removed the player from the power optimization list, the glitches evaporated entirely. Maybe something to look into, since it's a different cause than the three on your list?