Decreasing Volume

I have an 80g Classic which is, approx, 4-5 years old. I've notice recently that the volume appears to be decreasing by which I mean I have to set the volume at a higher level in order to be able to hear anything via the headphones (which also have their own volume control). Also when in the car I connect my ipod via an "aux" cable but i have to set the ipod volume at full "wack" as well as using the cars volume in order to hear it at all ( the radio's volume is much better). At the moment the problem isnt too bad because I can compensate using the headphones own volume control but I'd like to resolve the issue before it deteriorates further, i.e. the sound disappears altogether and I have to replace the ipod. Any ideas please?