> "Rockbox is open source software for portable music players" ?

I don't consider smartphones "portable music players" either.
So, to say it more drastically:
"Rockbox is an open source music player software for portable devices"?

This might of course be more misleading as it focuses only on RaaA.
So maybe ad another sentence of explanation below that states that it
can run both as an app on several portable device operating systems or
as firmware on the portable music players listed somewhere further down?

Just my two cents...

TheSeven