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.