Yes, you can achieve that!
The proper way to do this is to use JACK sound server, which is fully supported by recordMyDesktop. There is a nice GUI app for controlling it (though it's name is terrifying) -
You will want to use the Connections window of qjackctl, it will allow you to learn which audio input is which, and will enable you to direct both your system input & microphone input to your system speakers.
Once your JACK server is running, launch recordMyDesktop. If you use
gtk-recordMyDesktop you will be provided with a list of all JACK channels in the settings window (make sure to choose two with the Ctrl key in order get stereo sound!). If you run recordMyDesktop from command-line, you will need to experiment with command-line parameters, but that shouldn't be too hard.
Instead of routing all audio you want to record to system speakers, you might also experiment with different JACK connection schemes. Use it as you like. And note that there are many JACK connections tutorials for begginers, if case you found JACK difficult.
PS. You may find this article helpful: http://www.linuxplanet.com/linuxplanet/tutorials/6491/2