Updating the firmware will not have any effect. Your computer can only see files added while the player is connected in that same mode. Sounds like at some point (probably using Auto Defect mode) you transferred some music to it. Auto defaults to MTP mode whenever it can. Now it won’t connect except in MSC mode and it can’t ‘see’ those files.
I’d recommend formatting it (the player) and re-loading your content in MSC mode.
