> Would sound odd during calls, though!

I find myself thinking how you might dress or act to disguise it, and then this climactic (i.e. spoiler) scene from "Who Framed Roger Rabbit", and finally anxiety about the weird places my mind goes.

https://www.youtube.com/watch?v=BDrqCq4fGRE&t=73s

Although it's clear you are being facetious, let's see where it goes. I would surmise the best and most economical gas mix to breathe long term at ambient pressure would be somewhere between 80/20 and 75/25 mix of Argon/Oxygen.

I expect that the cost of the helium would dwarf the cost of of a fancy voice-shifting mic.