Virtual Microphone setup (eg. play mp3 into the microphone port)

Is there a way to stream audio into the microphone port without jacking the speaker audio cable into the mic port? Operating systems: Windows, Linux

Assuming you want a virtual solution -- for example, you want a) to use some software to play an MP3 on a computer, and b) to use some other software to record that audio output on the same computer, you want something like JACK:

JACK ... can connect a number of different applications to an audio device, as well as allowing them to share audio between themselves. Its clients can run in their own processes (ie. as normal applications), or can they can run within the JACK server (ie. as a "plugin").

See the JACK FAQ for how to use it, and the JACK download page to get the source tarball; a Windows installer is included. For Linux, your distribution's software repositories (apt-get, yum, etc) already include JACK binaries, so you can use your preferred package management tool to install it.

On Ubuntu, you want to install the packages "jackd", "jack-tools", and possibly "qjackctl":

sudo apt-get install jackd jack-tools qjackctl