Spell casting volume should be directly proportional to the level of the slot used.
Level 1? Barely above a whisper, like your character is trying to have a conversation while in a library or classroom and trying to not get caught.
Level 9? A thunderous roar. Completely overwhelms any other game sounds, blowing out your speakers and obliterating your real life eardrums.
Cantrips can start out quiet and grow as you reach levels 5, 11, and 17.
But the nature of spells is also very different. While the power level should be represented as well (e.g. more added sfx the higher you go), Meteor Swarm should sound very different from Astral Projection, which I would imagine to be more echoy and weird instead of just loud and smashy. Transcending through dimensions in ghostly astral form should be quite different from smashing meteors down from the sky.
