Usually, all programs send their audio output to your soundcard independently. With Virtual Audio Stream you can route the output signal of any application to be the input signal of any other application.
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MIXLINE Alternatives
MIXLINE is described as 'Audio mixing software that performs audio routing, mixing, and streaming. All bundled into a beautiful User Interface!' and is a Audio Router in the audio & music category. There are more than 25 alternatives to MIXLINE for a variety of platforms, including Windows, Mac, Linux, BSD and iPad apps. The best MIXLINE alternative is Equalizer APO, which is both free and Open Source. Other great apps like MIXLINE are VB-Audio VoiceMeeter, Audio Hijack, JACK Audio Connection Kit and BlackHole Audio Loopback Driver.