I have an 8 month old W580 which I use everyday as a walkman and occasionally as a phone. Since it is all that keeps me alive on painfully dull bus journeys, i was quite annoyed when i realised the volume in the left channel of the head set was slowly diminishing. after around a week a had almost completely lost the left channel, and the right channel was around half volume, will a great deal of lost quality. an example of this is available in the attachments. however, the full volume & channels returns when you gently push the bit that plugs into the backwards.
at first i thought that this was just the head set, so i tried another of my many (i keep losing the little in-ear pad things and have to buy new ones), and that had exactly the same results. Earlier today i tried my current handset with a friends W50, and i could here both channels at full volume/quality, so i think it is a problem with the actual phone, not the head set.
hope you can help :S