system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can musicians post cover songs on YouTube, but they can't release an audio-only version due to copyright laws?
gpt
Youtube has an agreement with every music publisher that allows people to upload otherwise infringing content to the site as long as Youtube transfers the monetization rights to the video to the appropriate publisher.  So when you upload a video containing a published musician's work, Youtube just automatically handles the monetization transfer without you doing anything on your end.

Its also possible for you to obtain a license to post cover videos on Youtube while retaining monetization rights.  There is something called a "mechanical license" that historically allowed people to play cover songs at live events.  Mechanical licenses don't apply to Youtube content and obtaining a license to play a cover song on Youtube requires negotiating directly with the music publisher.  

That isn't something that just anyone can do.  But if your channel has enough subscribers/views then publishers will work with you to license their content.

A mechanical license does allow someone to release an audio only version of their cover song.  Small Youtube cover bands don't release audio only songs because mechanical licenses cost money, which they're not going to make back if they're releasing the song for free.