this is achieved two ways, 1: they practice their set and set cue points on each track 2: some have a monitor setup in the room to act as the headphone, if you notice, you don't hear ambient noise in the room, it's because the audio out is mapped to the video source, so if the mic is off, you don't hear any sound in the room.
when I recorded a video of me mixing with my cell phone, you could hear the fader noise when I mixed or when I scratch.
they use an audio interface to record youtube and facebook videos: https://www.sweetwater.com/store/de...CwD5iu0R8G_cid8NffGxuWyzZ3NYqoThoC4HEQAvD_BwE
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