How to route audio output to any app on Windows 10

I needed to input audio to a browser on Windows 10. We were developing a HTML5 app and wanted to use audio to control it. Specifically, we wanted to route the audio we were listening to (from Spotify) to the browser (Firefox) for use as an input.

On Linux, it's much easier to route the audio you're hearing to whatever program. But on Windows 10, this is not always possible without some effort.

Some programs on Windows 10 allow you to choose the audio output as an input. An example of this is the Audacity sound editor / recorder. But browsers typically only allow you to choose the microphone as an audio input.

Step 1: Routing audio output as an input to a program

For this purpose, I found a piece of software called VB-CABLE.

I didn't need ASIO in this case, as I was just playing music from Spotify. For ASIO output you should try the HiFi-Cable software instead. ASIO is necessary for those who want to route sound from a professional audio DAW software such as Ableton Live, FL Studio, Cubase, Pro Tools or such.

After downloading and installing VB-CABLE, I ran the included control panel software and set the sampling rate. I used 44100 Hz.

I could now select the "CABLE Output" as a microphone input in Firefox. And there was a new output called "CABLE Input" to select in Windows (by pressing the speaker icon in lower right corner). By selecting these, the audio was now routed to the browser.

But now the audio was not routed to speakers at all. It would go directly to browser through VB-CABLE and nowhere else. I could see the audio controlling the visualization, but couldn't hear it.

Step 2: Routing audio to speakers as well

So we need to be able to route the audio from Spotify to two outputs: our speakers and the "CABLE Input".

For this purpose I found an app called Audio Router.

WARNING: the site at the address audiorouter.info is a highly suspicious page offering the same program. The site is hastily put together, the download link points to mediafire and there seem to be spelling errors. It is highly likely the site is spreading malware or viruses. You should avoid that site. The unfortunate downside of open source software is that anyone can make malicious versions of it.

After downloading Audio Router, run it and now you can duplicate some audio sources, like Spotify, and route them to several outputs at once.

Using this app it was fairly simple to do what we needed to. Although it seems to have some small bugs, it did the job in the end.