Basically a web link to the playlist in mp3 or m3u8 format can be played directly in notes from clients that support it.
Discussion
So I need to post a note with an Mp3 in order to share my song on there
Not necessarily. You can post a link from anywhere youtube, song.link, spotify etc of your music. From youtube it will render in some clients directly as they support it like nostrudel.ninja This is just a way to play directly in app instead of a redirect link to a platform. You can check stemstr.app and https://wavlake.com/.
Stemstr also renders directly in some nostr apps like this
ayyyyyyy