bitrate in kbps
The default is around 32-kbps per track with a mono-channel (VBR on by default)
The audio codec used is OPUS and the target sample rate is 48khz
510-kbps is the highest value allowed, with around 300-kbps the highest value for a mono-channel track.
When an audio bitrate is manually specified, CBR is enabled by default.
Setting the audio bitrate to be very high can sometimes cause video bitrates on weak connections to become stuck at around 30-kbps.