Requires ALSA, the Boost program options library and C++11 or newer.
Install and load the snd-aloop kernel module. With this module you can use a Loopback device, that captures the system output. See Module-aloop for an manual on how to configure the pcm. See asoundrc_example for an example configuration. (NOTE: you may need to change the hw id of slave.a.pcm "hw:1" in pcm.both to the id of your soundcard, use aplay -l for a device list).
Use ./avc -L loop -C default -P output -M Softmaster where loop is the loopback pcm, default the default capture device (usually the pc microphone) and output is the output pcm with the Softmaster mixer element defined in asoundrc_example.
See avc -h for a help menu.