Emulating an electret microphone with a DAC circuit
I did more research and learned that my original plan was a lot more complicated than it needed to be. I found that I could use frequency shift keying (FSK) like the one I saw here and here which are both for the iPhone and should be usable for other devices with little or no modification. I'm currently researching how to do it directly using C# code on the Netduino which isn't in the scope of this site anymore so I have to find help elsewhere.

Anyway, thanks to both Joe and Chris Stratton for providing answers. I'm sorry if I had to unset Joe's answer for my question as it does not apply anymore. Both answers are still very informative on my side though so thanks a lot. I'd probably make use of them in another project.