The media quality that you hear is entirely up to the sender's WebRTC implementation. If you're calling another Element Call user, we currently use the default audio constraints when capturing audio: https://github.com/matrix-org/matrix-js-sdk/blob/96ba061732b... - but if you wanted stereo, you could put { channelCount: 2, sampleRate: 48000 } or whatever into that line to achieve it.
