The default configuration of Multicam uses the sound from analog sources connected to the rear panel (using a Blackmagic Studio card). You can add other audio sources in the Hardware Settings:
Adding the source
Press then pick your source in the list on the right.
The source is then shown in your External Audio sources list:
You can use its settings button to configure the channels mapping for this source, but the default should be fine in most cases.
In some cases, you don't want to hear this new input in all the available outputs. Then you need to customize the mappings.
Press , you should see a pop-up similar to this:
For instance here, the Webcam audio input will be recorded and streamed, but it won't be present in the SDI output, and won't be heard in the audio preview / monitoring.