Check out projectM, "an open-source project that reimplements the esteemed Winamp Milkdrop": https://github.com/projectM-visualizer/projectm

If you simply drop in a song, it will render in realtime. You don't have to render it to a video. I am working on a "live" mode that will act much like Winamp, where you would select a playlist of songs and have it cycle effects.

Uneducated suggestion, feel free to ignore if inapplicable:

I don't know if this is common cross platform, but my Windows has an audio in device called "Stereo Mix" which is just all audio output played by all applications mixed together and looped back to an input device. If you could set your live mode to listen on any input device, then people can select Stereo Mix and use whatever existing software they have for playing/playlisting and have astrofox produce the visuals. You'd be a sort of Winamp-less Milkdrop!