The online teacher's Mac audio: balancing slides, video, and the call

When you teach live on a Mac, three sources fight for one volume knob: your slides, the video clip you share, and the call itself. macOS routes all of them through a single master output, so turning a loud clip down also turns the call down, and there is no built-in way to hold them at separate levels. The fix is a per-app volume mixer — free with Background Music, or fast with a keyboard-driven app like Fader — so a video sits under your voice, the call stays clear, and notifications never cut through.

Here is the moment every online teacher knows. You are mid-lesson on Zoom, Google Meet, or Teams. You share a short clip to illustrate a point, and it lands twice as loud as everything else — students wince, someone types "too loud" in chat. You reach for the volume keys, but that pulls down the whole class: now your own voice and the clip and the incoming questions are all quiet. You have one fader for a job that needs three.

The good news is your microphone is not part of this. Playback volume and mic level are separate paths, so nothing you do to the video changes how loud you sound to the class. The whole problem is on the playback side: slides, video, and the meeting's incoming audio all share one output that macOS won't split.

Why one volume key can't teach a class

Unlike Windows, which has had a per-app Volume Mixer for years, macOS exposes a single system output level. The keys on your keyboard, the Sound menu, and Control Center all move that one master. There is no native panel that says "Keynote at 100%, the YouTube tab at 40%, Zoom at 100%." So the instinctive move — grab the volume keys when a clip is too loud — is exactly the move that breaks the balance, because it scales everything at once.

For a teacher that is worse than for most people, because your mix changes constantly inside a single session: talk, play a clip, talk over it, pause, take a question, play the next clip. Each of those wants a different balance, and hunting for the right slider in the right window every time is friction you can't afford while thirty people wait.

Free fixes that get you most of the way

You can teach a clean class without buying anything. It takes a bit of setup and some discipline, but the tools are already on your Mac or one download away.

• Use each app's own volume. Most of your sources have an internal slider: a YouTube or web player, VLC, QuickTime, Keynote's media, Spotify for background music. Pre-set the clip's player lower before you share it, and it comes in under your voice instead of over it.
• Turn on a Focus before class. Do Not Disturb (or a custom "Teaching" Focus) silences notification banners and their sounds, so Slack, Mail, and Messages don't ping into a live lesson. Set it to turn on automatically at your class times and you'll never forget.
• Quit what you don't need. The fewer apps making noise, the fewer things share that one master. Close the music, the news tab, the game that autoplays.
• Install Background Music (free, open source). Background Music is a well-known open-source utility that adds a menu-bar volume slider for every running app. It's the free way to get true per-app volume — set the video app to 40% and leave the call at 100%, from one list.

If your specific pain is a loud clip drowning a meeting, the mechanics are the same as making music sit under a call — walked through step by step in how to make Zoom quieter than Spotify on a Mac. And if the culprit is a browser video, controlling YouTube volume independently of system volume covers the tab-level tricks.

Where the free route runs out

The internal-slider approach works until the moment you're live. Every app hides its slider somewhere different, some sources (system sounds, an app with no slider) can't be turned down at all, and a menu-bar mixer still means moving your mouse to the menu bar, clicking, finding the row, and dragging — five actions for a change you make while talking. Background Music also captures system audio through a virtual device, which is fine day to day but is one more thing to check when a class is about to start.

For teaching specifically, the cost isn't the setup — it's the reaching. When a clip comes in hot, you want to duck it in the half-second before students react, without breaking eye contact with the camera or losing your place in the slide. A slider you hunt for with a mouse can't do that.

The keyboard-fast way

This is the gap Fader is built for. It gives every running app its own level, like the others, but you reach those levels from the keyboard instead of a menu. Hit a toggle hotkey and your keyboard becomes a mixer: one key column is the video app, another is the call, another is your background music. Ride a clip down while you keep talking, snap it back up for the next one, mute a stray app — all with letter keys, no mouse, no window-hunting.

You can also set a mix once and recall it. Build a "teaching" balance — video under voice, call full, notifications muted — and it's there every class. For the apps you adjust dozens of times a lesson, that's the difference between owning a mixer and actually using it — and keeping alert sounds out of a live session is a keystroke away too.

Which approach fits your class

All three of the paid or open-source mixers set per-app volume correctly — this isn't a "which one can do it" question. SoundSource, from Rogue Amoeba, is the most feature-rich of the group, with a per-app equalizer and audio effects on top of volume; if EQ matters to you, it earns its price. Background Music is genuinely free and does the core job. Fader's whole reason to exist is speed: the fastest possible reach to the control, because a teacher changes the mix constantly and a one-time $19.99 with a 3-day trial is an easy call.

A setup that survives a live lesson

Whatever tool you land on, the reliable pattern is the same. Before class: turn on your Teaching Focus, quit noise-makers, and pre-set clip players a notch low. During class: keep the call at full, the video a step under your voice, and background music (if any) well below both. The only thing that changes lesson to lesson is how fast you can nudge those when a clip surprises you — which is the entire argument for putting the controls on your keyboard.

Bottom line

macOS gives online teachers one master volume for a job that needs three or four separate levels. Free fixes — each app's slider, a Focus for notifications, and Background Music for real per-app control — will get a careful teacher through a clean lesson. But live teaching is about reaching the control fast, in the second before students notice, without leaving your slide or your camera. That's exactly what Fader is built to do: every app on its own fader, all of them a keystroke away.