Spoofs a mobile user agent to trick YouTube Music into using the Opus audio codec if the song is not available in 256kbps AAC (High Quality), since desktop YouTube Music seems to be unable to play in Opus codec always defaults to AAC.
Opus is widely regarded as the more modern and better-optimized codec, especially for streaming applications. Its advantages are most noticeable at lower bitrates—such as YouTube Music’s Normal and Low quality settings—where its superior efficiency and audio preservation clearly outperform AAC.
At the High quality setting (256 kbps), Opus still offers theoretical benefits, such as better transient handling and fewer artifacts. However, at this bitrate, the difference becomes much more subtle and may only be perceptible to listeners with high-quality audio equipment and a trained ear. That said, it appears there is currently no way to make YouTube Music serve the 256 kbps version of Opus on desktop. YouTube Music only serves Opus at up to 128 kbps, even when High Quality is selected in the playback settings.
In practice, Opus at 128 kbps should be fairly comparable to AAC at 256 kbps for a streaming service like YouTube Music. In fact, AAC at 256 kbps may even have slight advantages over Opus at 128 kbps in certain scenarios.
I hope the day comes when YouTube Music on desktop gets fixed. It is completely ridiculous that YouTube video offers better audio quality than their service that is dedicated to music.