Record mp3 with FMS

Hi!

I'm trying to record .mp3 file with FMS. I can save .flv with my webcam (video+audio) but i'd to save audio only files in .mp3 format using only my microphone.
I just use netStream.publish(myFileName, "record") to save my files. I've tried netStream.publish("mp3:myFileName", "record") but it doesn't work. May i have tu create a directory different from /myapp/streams/_definst_/ to record mp3?

Thanks in advance! Any help will be useful and appreciate!
M.