My volvo's big central volume control has a large play/pause button in the middle and it works to pause anything that Carplay is playing. There's also a play/pause control on the steering wheel. Overall, my 2016 Volvo does a pretty good job with Carplay integration, though it also has a bunch of native software that mostly sucks in comparison, such as a voice control that I've never been able to use.

So it turns out that if you turn the volume all the way down to zero, it pauses playback. And if you turn it up again, it starts playing. I didn’t test whether navigation messages from Maps can still be heard. Not perfect as it takes a few seconds to do when you might need an immediate pause to avoid distraction, but it’s probably good enough for me.