An additional looping sound effect type for creatures would be nice. It would be good for creatures that don't make vocal sounds, but might make other sounds while moving. Like the slime, it could make a dragging noise while moving, but stop when it stops.
I suppose to reduce load on the client and server, any of these looping sound effects should be disabled if players aren't nearby or in the same area. I think that's how it works with regular sound effects.