macOS audio settings are scattered across four places that don't mention each other, with override rules Apple never documents in one spot. This is the map — where each setting lives, what beats what, and the answers to the "why did my mic change by itself?" family of mysteries.
The four places input settings live
1. System Settings → Sound → Input. The system default input. This is what an app uses when it doesn't have its own mic picker. The input level meter here is also your fastest diagnostic: if it doesn't move when you speak, nothing downstream will hear you either.
2. Inside each app. Zoom, Meet (via the browser), OBS, and most pro apps keep their own input selection, which overrides the system default. This split causes the classic confusion: you change the system input and Zoom keeps using the old mic, because Zoom was never following the system setting. Rule of thumb: check the app's picker first, system second.
3. The browser. Web apps add a third layer — Chrome and Safari have per-site microphone permissions and a preferred-device setting. If Meet mysteriously uses your webcam mic while everything else uses your desk mic, look in the browser's site settings, not macOS.
4. Audio MIDI Setup. The app Apple hides in Applications → Utilities is where the real controls live: per-device sample rates, channel formats, and Aggregate Devices. If a mic works everywhere except one app, or sounds pitch-shifted, sample-rate mismatch is the usual cause, and this is where you check it.
Why your input keeps "resetting"
macOS quietly switches the default input when devices appear or disappear. Plug in almost anything with a mic — a USB interface, some headphones, certain webcams — and it can grab the default. Unplug your mic and reconnect it, and it may return to the bottom of the priority pile rather than reclaiming its place. AirPods are the most famous offender: connect them for music and your calls may silently start using their mic, which drops your audio quality to headset-radio the moment the mic activates.
There's no supported way to pin a default input across all of this — which is why part of the remote-work population re-checks their mic dropdown before every important call, like a nervous tic.
Volume, gain, and the meter that lies
The Input volume slider is a digital gain applied after the mic. Cranking it to compensate for a too-distant mic amplifies room noise by exactly as much as voice — loudness without clarity. If you need the slider above ~75% to register, fix distance first (the placement guide covers the numbers), then set gain so normal speech peaks around two-thirds of the meter.
The layer that makes the pickers stop mattering
Every mystery above shares a root cause: apps bind to one physical device, and physical devices come, go, and get stolen by the OS. The way out is a stable virtual device that never changes while the physical mics behind it do. That's how SoundMesh sits in this stack: every app — Zoom, the browser, dictation — selects "SoundMesh Virtual Mic" once, and which physical microphone is actually live at any moment is decided continuously behind it, based on which one hears your voice best. Plug mics in, unplug them, add AirPods for audio out; your apps' input never changes again.
Bookmark this for the next time a call app plays dumb — and if step "check four settings panels" has gotten old, make it one setting, permanently.