Try to query about technical aspects of this from Cypress or ST.
I know the mass storage firmware solution seem to be dealt with confidentiality since I have met some Taiwan vendors who sells the mass storage solution (as a package device with the MCU firmware built in) but never they can share the firmware implementation details to us.
Atmel has a 8051 derivatives which has a MP3 decoder built-in. The kit provides sample source code to build a MP3 player, which also include Mass Storage Class code.