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.

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.

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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 seas