Slimes and magma cubes won't play their respective attack audios unlike Java Edition. It because of a coding bug inside sounds.json.
How to fix: sounds.json is wrong and one of the slime sound events say "mob.attack". Change it to "mob.slime.attack" and it should work. That's it. Only change the sound event name. I put that in a resource pack and the sound works now.
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We're tracking this issue in MCPE-184043, so this ticket is being resolved and linked as a duplicate.
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