if you play a custom /playsound (ui category) inside functions, it wont play sometimes. But it works fine everytime you type it in a chat like this:

/playsound all_plains @s[scores=\{music_tick=1}] ~ ~ ~ 0.3

in the video you see an example what the meaning is for the function. everytime you enter a so called "new location" at the sidebar, it plays a new /playsound.