You want the same audio playing in two places at once: speakers for the room and headphones for you, or a soundbar plus whatever is capturing your screen recording. macOS doesn’t offer this as a simple toggle. The Sound menu lets you pick exactly one output. That’s not a missing feature so much as how the system was built, and getting around it cleanly means understanding why it works that way first.
Why macOS only sends sound to one place
Core Audio, the layer that moves audio between apps and hardware, treats every physical output — built-in speakers, a USB interface, Bluetooth headphones — as a separate device object. The system mixes every app’s audio into one stream and hands that stream to whichever device is set as the “default output.” There’s no native checkbox for two at once, because the mixer was designed around a single destination.
Apple’s workaround lives in Audio MIDI Setup, a utility most people never open. It lets you build an Aggregate Device, or specifically a Multi-Output Device, which bundles several physical devices into one virtual one that mirrors the same signal to all its members. The system then treats that bundle as if it were a single output, which is the trick that makes two-at-once possible at all.
The catch is clocking. Every audio device runs on its own hardware clock, and even when two devices both claim 48 kHz, their real clocks drift by a few parts per million relative to each other. Over a few seconds that’s inaudible. Over a few minutes, two independently clocked devices playing “the same” signal walk out of step. A Multi-Output Device designates one member as the clock master and can apply drift correction, via resampling, to the others — that’s what the small checkbox in Audio MIDI Setup is for. Bluetooth devices make this worse: they don’t expose a real hardware clock over the air at all, just a compressed packet stream with its own jitter, so the correction is coarser and more likely to produce an audible tick or a brief mute.
Your options, from built-in to dedicated software
The Multi-Output Device in Audio MIDI Setup. Open Audio MIDI Setup in Applications > Utilities, click the plus button, choose Create Multi-Output Device, check the two devices you want, and enable drift correction on every device except the master. Select the new device in Sound settings and you’re mirroring. This works well for two wired devices, like built-in speakers plus a USB DAC. It’s rougher with Bluetooth for the clocking reason above, and you lose independent volume — both outputs now share one system fader, and changing one device’s level means going back into Audio MIDI Setup.
Third-party menu-bar utilities. Several small apps wrap the same Aggregate Device API in a simpler menu, mostly saving you the trip through Audio MIDI Setup. They inherit the same clock-drift ceiling, since they’re built on the same underlying primitive Apple provides — they’re a shortcut to the same mechanism, not a different one.
A system-wide audio app that sits earlier in the chain. Instead of fanning a signal out after Core Audio has already picked one destination, a driver that captures audio before it reaches any physical device can duplicate the processed stream itself, without relying on Apple’s Multi-Output drift correction at all.
Doing it with ZonaReq
ZonaReq installs a Core Audio driver that captures system audio at the top of the chain, so it can hand a copy of the processed stream to more than one output directly.
- Install ZonaReq. It requires an administrator password and a restart of the audio service — the same category of one-time setup as building a Multi-Output Device, just handled for you.
- Open the output panel and turn on every device you want playing the mix, for example your speakers and a pair of headphones.
- Each active output gets its own render pass from the same captured buffer, rather than being chained together after the fact, so there’s no master/drift-correction relationship to manage between them.
- If you want different levels per output or per app instead of one shared system fader, use the per-application mix or that device’s volume section, rather than fighting the single Sound settings slider.
What this doesn’t fix
Bluetooth headphones carry their own latency, baked into the Bluetooth audio stack itself before any Mac software gets involved. If you mirror sound to a Bluetooth device and a wired one at the same time, the wireless one will always lag slightly behind — no driver, including ZonaReq’s, can make a wireless codec and radio faster than they are. For anything that needs tight sync between two outputs, use two wired devices, or accept that one path will be exactly in time and the other close but not perfect. And mirroring is not the same problem as sending different audio to each device — that’s multi-zone routing, a separate use case this approach doesn’t cover.
Co-Authored-By: Claude Sonnet 5 [email protected]