Audio Interfaces allow you to connect things like microphones and guitar cables to a computer and record via USB most commonly. It also has outputs typically and allow Studio Monitor Speakers, computer speakers or headphones to listen to the recorded or real-time incoming audio to play through the computer.
Few things are as important as great audio when live streaming. Many people run live streams in the background while they work and so for them, audio is perhaps even more important than video. Whether it’s a microphone, an instrument, your game audio, or some combination of any/all of them having great audio is key to having a professional stream.
Interfaces
https://www.bhphotovideo.com/c/product/1539514-REG/rme_babyface_pro_fs_24_channel_192khz_bus_powered_usb.html?sts=pi&pim=Y
https://www.bhphotovideo.com/c/product/1479275-REG/focusrite_scarlett_2i2_3g_scarlett_2i2_usb_audio.html?sts=pi&pim=Y
Invisible Ministry
http://www.invisibleministry.com/