This program is a simple demo sending PPM over an audio cable for the Spektrum DX7 and friends.
Feel free to modify to generate the PPM you need.
You may record a sample PPM input from your RC Radio to find the proper amplitude (it won't work without!)
Many radios however work with any amplitude.. then again the DX7 does not.

There's more info on http://www.insecure.ws/

Oh yeah.. you'll need gtk, pygtk, pyalsaaudio, pygame, alsa and python to run this program.
Just type ./ppm.py to start. If it complains, it should tell you what's missing to run.

The code isn't especially nice, I threw it together quickly (yeah, never heard that excuse before did you?). Nevertheless, it works fine as a demo.