Audio MIDI Setup is not in System Settings. It is an application, in /System/Applications/Utilities, and it is the only place on a Mac where several genuinely important things can be done at all.

Opening it

From the Terminal, or anything that can open a URL:

open -b com.apple.audio.AudioMIDISetup

Otherwise: Finder → Go → Utilities (⇧⌘U) → Audio MIDI Setup. Spotlight finds it too, if you remember the name, which is the part people do not.

It opens on one of two windows, and the Window menu switches between them: Audio Devices and MIDI Studio.

The Audio Devices window

Every input and output your Mac can see, with the things System Settings does not expose:

• Sample rate and bit depth, per device. The Sound pane has no equivalent.
• Channel layout — which physical output is front left, and so on, for interfaces with more than two.
• Per-channel volume, separately from the device's overall level.
• Input gain, on hardware that publishes it.

The gear menu at the bottom is where devices get configured, renamed, set as default, or built.

Aggregate device or multi-output device

These are different tools and choosing the wrong one wastes an afternoon.

An aggregate device combines several interfaces into one device with all their channels, so a DAW can use inputs and outputs from more than one piece of hardware simultaneously. Two 8-channel interfaces become one 16-channel device.

It needs a clock source — one device is the master — and drift correction switched on for every other device in it. Without drift correction the devices run on independent clocks and slide apart, which sounds like clicks at first and like a sync problem later. Switch it on for everything except the clock master.

A multi-output device sends the same audio to several outputs at once — speakers and headphones, or an interface and a streaming device. It is for playback, not recording.

Its well-known limitation is worth knowing before you build one: a multi-output device has no volume control. The keyboard volume keys stop working, the menu bar slider does nothing, and levels have to be set on each member device individually. That is not a bug and there is no setting for it; a multi-output device is a fan-out, and there is nothing there to turn down.

The MIDI Studio window

Window → Show MIDI Studio. A map of every MIDI interface and device the Mac knows about.

Two things people come here for:

• Rescan, when an interface has been connected and nothing can see it. Devices that were online once but are unplugged stay in the window, greyed — clearing out old ones is worth doing when the window has become a museum.
• The IAC Driver. Apple's virtual MIDI cable, for routing MIDI between apps on the same Mac. It ships disabled: double-click it and tick Device is online, and it appears as a MIDI port in everything. Add ports if you want more than one independent bus.

This window is also where a multi-port interface's ports get named, which pays for itself the first time a session has three of them.

Why the sample rate keeps changing

A recurring annoyance with a dull explanation. Several things can set it:

• An app took the device and set it. A DAW opening a session at 48 kHz will set the interface to 48 kHz, and leave it there afterwards.
• The device was unplugged and reconnected. Some interfaces come back at their own default rather than at what they were.
• Something is resampling quietly. If an app is playing 44.1 kHz material through a device sitting at 48 kHz, macOS converts it and says nothing.

There is no system-wide lock. The practical habit is to set the rate in Audio MIDI Setup for the session you are about to do, rather than expecting it to persist across a reconnection.

The volume problem this window does not solve

Audio MIDI Setup will show you every device's level, and let you set it. What neither it nor System Settings will do is give you those levels somewhere you can reach them quickly.

macOS has one volume control and it follows the current output. The keyboard keys, the menu bar slider and the Sound pane all act on whatever you happen to be listening through right now. Every other device on the machine has no control at all — so turning the monitors down while you are on headphones means switching output, adjusting, and switching back.

Audio MIDI Setup is the place you can do it, which is better than nothing. It is just not somewhere anyone wants to go mid-session.