just solved it! Aparently there is an issue with my Logitech dock that makes iOS 5 do really strange things. It can start playing music without my command for example. Or sometimes the speaker/volume icon on the screen won't disappear after I remove it from my dock.
My solution: restart the iPhone. Everything back to normal. iPhone 4s finally playing all my music as it was intended to do. Really thought this was a new issue.