VCV Rack is an open source virtual modular synthesizer, which is software that can be used to create synth music or sounds. The interface is modeled after physical synthesizers called Euroracks. Recently, Jon Williams has ported rtl_fm to the VCV Rack plugin system which allows him to include live FM music in the synthesizer output. This can be used to create some interesting sounds which he demonstrates on the video that is shown below. The frequency tuning is 'voltage controlled', which simply means that it can be adjusted with a knob in the software.

The code is available on GitHub and is fully open source.