SOMAFM's SF police radio with ambient music is surprisingly good for concentration, I have this on an alias in my box : mpv https://somafm.com/sf1033.pls

Does nothing for me ("Play" does nothing) with uBO enabled in FF.

Works fine for me in Firefox for Android with uBO.

The music is quite sparsely melodic though and easy to miss

Maybe because I'm not signed in? I disabled uBO and it still doesn't play. Hmm.

SomaFM is some of the best of the internet. There's no ads and you don't need to sign in. You can just stream it without faff.

Try opening your browser's console, network inspector, etc. and see what happens when you try to play. There will almost certainly be enough hints in there to diagnose the issue.