If the mp3 is hosted somewhere already, the shared link to mp3 plays directly in amethyst, snort, etc. A lot is shared like that over the mostr bridge or like this:

nostr:nevent1qqsgtle5rdvgu0wty74mwenyzknphgnzp96dl74fs8gc4du6hagu9egpz4mhxue69uhhyetvv9uju6rvd3hjumrfwejsygr7tljgmmsjeqxt2aqr6uzkcqw9mmevnrk222eatlfakylu3rre6qpsgqqqqqqsce7u3p