Step by step

On the input thumbnail, check that the speaker is 🔊 (enabled). If it shows 🔇, click it.
In the mixer, check that the channel is ON, not MUTE or OFF.
Check whether the channel's AFV is on: with it, the sound only opens when the input is on Program.
If the meter doesn't move, check the audio device in Settings › Audio.

Audio disabled

The thumbnail speaker enables or disables the input's audio everywhere. When disabled, the sound plays nowhere: not on Program, not on Preview, not in the headphones, even with the mixer on. In the mixer, the channel button shows OFF.

ON, MUTE and AFV

In the mixer, the channel button shows ON or MUTE. With AFV (audio follows video) on, the input turns ON when it goes to Program and MUTE when it leaves. For videos, the audio always follows the video. If a microphone needs to stay open the whole time, turn off its AFV.

Microphone permission on Mac

If the Mac denied the microphone, adding an audio device shows the permission warning. Click Open Settings and turn on NWK in System Settings › Privacy & Security › Microphone.

Device unplugged or changed

In ⚙ Settings › Audio, the AUDIO INTERFACES (INPUTS) list marks a device that left the computer as (disconnected). Plug the USB cable back in. In the audio outputs, device missing means the same thing.

You don't hear it in the headphones, but the meter moves

You choose what plays in the headphones: press H to switch between Preview, Program or both. Solo and PFL only change what you hear in the headphones, not what goes on air.

Check before going live

In ✓ Check before going live, NWK listens to the microphones for 3 seconds. If a microphone stays silent, the Audio group warns you on its line: no sound in the last 3 seconds: microphone off or nobody talking?

No sound or the meter isn't moving