Leandro is a piece of software that explores different methods of audio synthesis and different audio filters, written entirely in C++, using portaudio and Qt.
It synthesizes notes from MIDI files using its own instruments and audio filters. After importing a .MIDI file, the user can choose from the different synthesizing methods and combine any number of filters and the melody will be generated accordingly.
Everyone loves images, so here's a screenshot of Leandro's main screen in action:
Leandro also supports multi-channel MIDI files, and allows for each channel to have its own synthesis method and assortment of audio filters:
After experimenting with different "instruments" and filters, you might like what you hear: the session's recording can be started and stopped at any point, and .wav files can be saved with the contents of those recordings.