Could be an option, but we take high audio quality as a point of pride and encode in Opus 128k by default. Audio doesn't only include speech but also any sound effects, media present in-world, etc.
But that might be an interesting experiment. Right now the low cpu usage/high quality/faily high bandwidth usage category is something we're looking to have an option for.
I appear to be able to get maybe 30% better performance -- pretty nice, but not nearly big enough especially on low end servers.