May I also note that, in addition to registering a singular sound using "MOD_ID:soundname.ogg",
it is also possible to use randomized sounds using
event.manager.soundPoolSounds.addSound(MOD_ID+randomSound1);
event.manager.soundPoolSounds.addSound(MOD_ID+randomSound2);
event.manager.soundPoolSounds.addSound(MOD_ID+randomSound3);
where randomSound1, randomSound2, randomSound3, etc. are (different) files in assets/MOD_ID/sound/
and then play them using
playSoundAtEntity(Entity, "mod_name:randomSound", float, float);
which plays one of the above registered number-terminated sounds, at random, each call.
Just thought I'd mention it for completeness' sake.