Ok I understand now. That is very easy to solve:
Go to the Samples folder in your main IL2 directory. In there you'll find 5 other folders: Crash, InFlight, Intro, Menu and Takeoff.
Crash: plays music when crashing airplane (after being shot down f ex)
InFlight: Plays music while flying
Intro: Music for splashscreen
Menu: Music for main menu in game
Takeoff: Plays music when on runway before take off.
I guess its the InFlight, Takeoff and Crash folder that interest you. Just take music (*wav file) and throw them into the folder you wish. In game, in the Sounds and Music menu, enable to play sounds InFlight on Takeoff and during Crash. (you'll find some buttons there )That's it!
Btw: Putting radio chatter *wav files into the InFlight folder will make you hear radiochatter during flight ! This is a very nice option. You can find some radiochatter files to download here on SOH or on other sites, like
www.mission4today.com
Enjoy !