...when played on your laptop?
It's possible that some of the playback circuitry within the YP-Z5 has degraded over time, though that seems a bit unusual given what you've described about its recent usage. When you put away the player for its 'vacation', was it stored in a (relatively) safe, dry location? No chance of it becoming damaged from static discharge or anything else similar that could harm the unit?
I have had my MP3 above for a good few years and haven't used it for same then a few weeks ago I thought I would use it again, so i plugged it in to my laptop using the usb cable which is using windows 7 and media 10 - transferred the music files accross by MTP everything looked absolutely fine, could see albums, their covers, song titles etc. Then I listened to it I can only say that it sounds squashed, you can hear the music but not the singing and the music sounds distorted I have changed the headphones 3 times and its really weird, I can find no one who has experienced it let alone find out how to fix it, I would appreciate anyones help please.