A number of things are required for the radio:
The difficulty option "Realistic Navigation" needs to be on.
You need to make sure the mission you're playing actually has a radio tower for the station you'd like to play.
You'll also need to supply your own music. IL2 is very picky about what it will play, you'll need to convert MP3's to RIFF header WAV's.
When all that is taken care of, check the control assignments for next/previous beacon and map them if necessary. Use those keys to select the desired radio station.
If you need more info, check the guide for 4.10.
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