Don't forget that the iPhone only has one speaker, the other port (looks identical) is a microphone. Nothing wrong with your iPhone.
I had a problem that sounded like a speaker kicked the bucket on my ipod touch 4th gen. It sounded like a vintage radio, cool but really annoying. I did an update for it through iTunes and that fixed. If you haven't done this recently or don't know how, in itunes click update and itunes will take care of the rest for you