I just spent most the night researching this for a friend. It seems the cm10(i think it's 10) had this bug in it, and fallen is based on that version. I haven't tried anything yet, but here are some things that I read that might work: enable lock screen sounds, flash newer radio, disable all other apps that may conflict with sounds. I'm going to be trying this on my friends device as soon as I can. If not, flashing to a different rom . 
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