As posted by others, i agree a little gentle press near top ear speaker does fix the problem. that big question is why?
when u see inside, u will find black speaker which has two brass type spring connections. the two screws with steel bracket hold the speaker pressed down.
I believe, this is classic case of spring load loss over time. all type of springs loose load over the period of their life . so when kept pressed for long time, they loose tension. I could be wrong, but i believe, this is the issue. you can't over torque screws because of limited their length. if you tighten two screws and it fixes issue, you r lucky. I tried, it worked for few weeks for me.
I am planning to put folded paper in-between speaker and metal cap. I think that will put extra compression and i will push speaker down against metal cap. I may try that after few weeks. then last but not the least, play/extend then brass springs..that may prove expensive as i may damage speaker-part in the process. i keep my fingers crossed.
let me know what you think !!