/playsound wont play custom sounds in the resource pack.

the /playsound doesnt play custom sounds defined in the resource pack, (for example: an resource pack has:

resourcepack/assets/minecraft/sounds/random/countdown.ogg, and /playsound random.countdown @a doesnt work.

But if your .minecraft folder has:

.minecraft/assets/sound/random/countdown.ogg, then the command will work!