If you do not want to stream voice from a microphone or external audio source, but rather stream audio from another application on the same computer, there are several ways to do this.

1. Internal audio routing software

You can use Dante Via or another "audio router" that will send the audio from an application on your computer to the web browser. In this way the browser understands the signal of your choice as an audio input.

>> See details and explanation here.

2. Loop trick

If you don't want to install a special software, you can also do the trick manually. For this you set up an "audio cable loop": You simply take a 3,5mm audio cable, and plug it in both the output and input of your computer, so basically the computer "hears" its own audio signal.
If your computer does only have one audio jack for input/output, you can simply get an external USB sound card that offers separate in- and outputs. You can get them for under 10 Dollars, like this one here on Amazon

3. Patchbay by LiveVoice

Patchbay is a desktop app by LiveVoice that allows you streaming several input sources at the same time with only one computer. But it can even do more: You can choose any audio file on your computer and play it through Patchbay, streaming it directly to LiveVoice.

>> Learn more about Patchbay here.

Using a second computer/device

In some settings the best way might be to use two different devices. You play your audio on one device and have a second one connected via an audio cable acting as sending device. The advantage is that you have a separate device/screen to check everything - for example when doing video projection or need the computer for other applications at the same time.