I'll try to keep this feature request short and simple.
When the Emby server needs to transcode audio to make it compatible for the Emby client, it will use a hardcoded value for the bitrate that cannot be changed.
For example, if the audio gets transcoded to Dolby Digital AC-3, it will always use a bitrate of 384 kb/s. This value cannot be changed, even though Dolby Digital AC-3 allows a maximum bitrate of 640 kb/s. Although 384 kb/s is a decent compromise between quality and data, it is very compressed and the quality is far from being transparent.
The same thing applies to every other audio codec the Emby server may transcode to, such as AAC.
So the request is simple, allow users to specify the bitrate that should be used for transcoding audio.