Even if they accept patches, contributing still sounds like a bad deal. It's free labor for some company. FOSS at minimum means the right to fork, precisely what "source available" seeks to deny.
Leaving aside the question of winamp vs comparable programs, does anyone even care about desktop music players any more? I'm a throwback and use command line players, but I thought the cool kids these days use phones for stuff like that.
I understand there is some technical obstacle to porting Rockbox to Android, but idk what it is and haven't tried to look into it.