As u said when u r going to upgrade they said there is no updates available then u have latest version of firmware. 2nd thing i don’t think so that older version don’t let u play mp3 or transfer mp3 if new firmware version comes. check out those mp3 files which u r trying to transfer try to transfer same files on other W800 if its works in that mobile then give your cellphone to Sony service shop they will short out it. If u know each and everything about cell phone software and firmware installation and other stuffs which have to do after firmware update then do it by yourself or lese just give it to related Authorised Service Station.
But i think firmware is not problem for your problem.
March 7th, 2010 
fix 
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As u said when u r going to upgrade they said there is no updates available then u have latest version of firmware. 2nd thing i don’t think so that older version don’t let u play mp3 or transfer mp3 if new firmware version comes. check out those mp3 files which u r trying to transfer try to transfer same files on other W800 if its works in that mobile then give your cellphone to Sony service shop they will short out it. If u know each and everything about cell phone software and firmware installation and other stuffs which have to do after firmware update then do it by yourself or lese just give it to related Authorised Service Station.
But i think firmware is not problem for your problem.
no new updates yet