macOS Bluetooth headphones connection I use a set of Bluetooth headphones (Bose QC35) that can connect either with good audio or with a microphone and some distortion. This works well with my iPhone usually in the audio mode, but switches when calling or answering the phone.

On my Mac (macOS Sierra 10.12.4) they work as audio only.

How do I switch the headphones between modes?

On audio only they do not appear in the System Preferences -> Sound output.

On odd occasions I have made them appear in degraded audio and microphone mode by moving out of range and losing the connection and then moving back in range but this does not occur reliably. In this case the headphones do show up as a microphone in System Preferences.
I do know that the degraded quality can be fixed by switching the Mic (Input device) to your Mac's mic rather than the headphone's mic. It's a software issue as far as I know.