Sound Quality from Siri Voice Feedback in Workout App Really Poor

I think I didn't explain my suggestion properly. As you say, guessing and that is not always too helpful. I am suggesting to test each stage of the audio chain by changing one thing at a time, and using easily reproduced sound sources like music or a YouTube playback which can be run several times to compare can identical source over different routes.

As I understand it, the link from your phone over your car audio is probably Bluetooth but the audio is injected into the final audio stage before output to the speakers, the other source (DAB, FM, whatever) is unimportant because it probably gets muted for things like Maps directions.

The other thing which would be useful to compare is to listen to your AirPods Pro on both the Apple Watch and same (or similar) audio from your iPhone. That will help you decide whether the problem is the incoming BT handling in the AirPods, or the outward signal from your watch.

The first thing I would compare is the corresponding headphone quality for phone or FaceTime voice, and for music playback. Also compare with using the phone’s speaker instead of Bluetooth. That should tell you whether the problem is in the Bluetooth stage (maybe even the headphone) or the underlying audio from Siri.