Soulseek is the only "FOSS" solution. It's literally piracy though. There's no real free or open source way of getting licensed music. That's the whole point of copyright and royalties.
I buy physical media (CD, vinyl, cassette). The CDs get ripped to flac and I use Soulseek to download flac versions the of vinyl and cassettes. I listen to the physical copies at home and use Plex/Plexamp when I'm out.
Congratulations! Depending on the country, you can either be considered a pirate or be in a legally gray area.
The Nicotine+ client is FOSS, the Soulseek network is not though. Just a clarification. It's still the only relatively widely used means of obtaining music besides torrents, even though people can share any kind of files, not just audio ones.
With FOSS, self-hosted, Subsonic-compatible music media servers like Navidrome, one can build a streaming-site alternatuve with one's own collection.
I'm not sure there is any. The nature of copyright makes it so it's impossible to stream music without paying a lot of money to the three or five megacorporations that own all the music in the world.
The only FOSS way to do it will be by sailing the high seasI use Blokada on Android and it blocks all the ads I would normally get on Pandora.
Invidious might serve depending on your needs.
Jazz flute.
Skin flute
If you're talking about streaming your own music, you could set up Jellyfin and then do all of the work to safely put it behind a reverse proxy and access it remotely.
It's a lot of work though, and you don't get new music. Still, that's about as good as you're going to get if you want a legal FOSS option (others have posted illegal options if you're looking for that.)
Navidrome is also a nice self-hosted option for music streaming. I recently cancelled my Spotify subscription in favor of hosting my own music library on it.
On Android, I like Spotube. You can sign into Spotify if you want, and the app will interact with it, but uses YouTube as the source for streaming the audio. This is good if, say, you have a free Spotify account and aren't at your PC to use the web player + uBlock.
I keep a folder with all my music and sync it to all my devices with Dropbox. Yes.
