Paul Kafasis from Rogue Amoeba has posted about this pretty interesting little improvement in the new Macs with the T2 chip that Apple currently ships:
On older Macs, the headphone jack and the internal speakers are essentially separate ports on a single output device, and only one of these ports is allowed to be active at a time. Because of this, audio can be sent to either the built-in speakers, or the headphone jack, but not to both. As well, if anything is connected to the headphone jack, the OS shuts off the built-in speaker completely.
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.
I’m not sure how practically useful this is, but it sure is interesting. I wonder if I can enable two simultaneous audio outputs, where one output is the built-in audio jack and the other is a USB-C to 3.5mm dongle.