Which file to extract?

[TkSilver]

first you will need to extract the music.yaf files (probably it looks to be the most likely of the sound files) -- in the USA version it is in the userdir\sound (probably the same in other versions but I don't own any of those to test)

From there you will need to extract the individual files (probably in the at3 format) from the yaf container. there are tools to do that (not sure if ppsspp has any built in) google is your friend.

After that you will need to convert the at3 file to something not sony controlled and unplayable through most means, again google will help you find one.

Then the rest is just listening to the resultant files to find the music you want.