Hey guys!!.. i have HTC P3400i!! pretty decent phone but i am facing a little problem...i hope u all can help me!!

my problem is when my phone is fully charged and when i play music then the speed of playback is too fast as compared to normal speed!! more over the volume is very low when it is fully charged as compared to volume and speed of playback when it has 50% or less battery!!.. Strange problem!! i am unable to figure out!!
plz gimme a solution if u know!!
Thnx and gud day!!..