Apple iPod earphone problem
Sounds like to me that your earphones have started to prematurely break down.  How does the cable look?  Is there bends or twists near the ends?  If there is then perhaps a wire is broken inside the cable.  Normally this would mean no sound rather than a decrease in volume.  Either way it seems that the earphones will need to be replaced completely.

Are you using the apple in ear earphones?  If so then simply replace the steel mesh inside the earphones by simply unscrewing them.  If you lost your extras you can find them on Amazon through a third party source.