My internal speaker has just recently started playing at the same time as the headphones, and the headphone noise sounds tinny. I'm considering sending it back to Apple for repairs unless there is a fix.
I had music playing through internal speakers when plugged into headphones and my car connection. In my circumstance, it was not due to loose connections. After much searching to no avail, I finally looked at my owner's manual, I found that a complete reset fixed my problem. Hope that this may work for you.