Choice of file format

[Christopher Jones]

I assume you're running under Windows
Have you tried not forcing any particular format and using just the command 
line ?
My "radio only" messages come down as mp3 without any intervention.
e.g.
get_iplayer --type=radio -g 10068
creates
"A_Point_of_View_-_Book_Choice_b00tmtft_default.mp3"

....however I can see that if you use Web PVR Manager to do this you will 
receive an *.aac file