Two years ago, YouTube introduced a Premium 1080p video streaming feature that plays videos at high bitrates. In the future, a similar feature will be provided for audio, giving users more flexibility.
Android Authority recently found evidence through the YouTube app code. The line of code “persistent_audio_quality_auto_description” was found with three audio quality options: Auto, Normal, or High. As of today, the audio quality of YouTube videos uses the Opus 251 codec which is played at bitrates of 128kbps and 48KHz. The overall quality also depends on the editing and audio recording equipment of the video owner which will then be processed by YouTube.
This audio quality selection feature is expected to be more or less the same as for videos, perhaps High is for higher bitrates only. While Normal is with Opus 251 and Auto according to the speed of the user's Internet connection device. It is not sure when this feature will be offered but what is certain is that it is limited to YouTube Premium subscribers only.