Macs With Two Built-In Audio Devices
With these new Macs, there are actually two distinct output devices. The headphone jack and the internal speakers are separate devices, completely independent from one another.
This change means it’s possible to send different audio sources to each outputs.
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In the course of researching this, I asked friends and colleagues to test several other recent Macs. It appears that distinct output devices are also present in both the MacBooks Pro Apple released in July of this year and the iMacs Pro which started shipping at the end of 2017.
Update (2018-11-16): Rob Mathers:
The ability to switch audio outputs while keeping a pair of headphones connected is something I’ve thought Macs should have ever since I got my first MacBook Pro.
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Didn't Mac Pros have this feature ten or more years ago already?
Yeah I’m sure I recall having to be careful on late night editing sessions to make sure the speakers were disabled after plugging in the headphone jack on on the Mirror Door PowerMac G4 - ocassionally it wouldn’t do this as expected.
Yep, on my 2018 MBP I'm sending alerts and system sounds (mostly incoming mail sound) via the built-in speakers, and the rest goes over the headphone jack. Pretty neat. I'm also using a small shell script from the Touch Bar via BetterTouchTool to switch output sources with the SwitchAudioSource CLI. Settings tend to stick so far.