|It sounds like the headphone jack is going bad. |
There is a switch in the jack itself, when you plug in the headphones, the switch turns off the external speakers. The switch may be going bad thus causing the speakers not to come on when you remove the headphones.
Also the audio problem you describe sounds like what happens when you take the positive lead from both channels and use that for the signal (instead of using the + and - leads), you get only the signal that is in stereo and it cancels out anything in mono. Usually the vocals are recored in mono (which is why they sound quiet) and the instraments have some sort of stereo effect to fatten them up so you are hearing those at normal volume. This can be caused by a bad headphone jack