I am not sure if anybody has posted something like this, so here you go.
Dont like the music? Add your own music.
Get you favorite music (I'm using Earthshine by Rush), covert it to .wav format.
Now go to to you FSX folder, find the folder "Sound", open that folder and look for the "UIMusic" folder.
Drop your new music in to that folder.
Now just simply rename the new music as FSX##. Replace the ## with the next number (IE, my new one is FX09).
Start up FSX and change the music to the numer of the new wav file or, if you are bold, change it in the FSX.cfg.