An issue with bluetooth voice call quality in a Honda Clarity

First of all, try turning down your vehicle's volume; I've noticed that whenever I am calling any sort of automated switchboard prompt that takes verbal responses, the prompts always get interrupted as if they've received a response before I ever say a single word. Usually lowering the volume will fix this, but the volume will be inhibitively low so I just end up switching over to the device speaker, though.

Regarding the audio quality on the other end, here's my theory: I'm fairly confident it has to do with the BT profile in-use by the HMU. This isn't my area of expertise so take it with a grain of salt, but I believe the HMU switches the audio band over from A2DP over to HFP during calls (or something to that effect). My understanding of it is that one is meant for audio media streaming and playback, whereas the other is for telecommunications; for whatever reason or limitations, the HFP profile has terrible sound quality and results in a constant background hum/hiss. An example of why I'm inferring this would be if you are on your laptop and you go to, say, Google Voice and try selecting the device/speaker options. Notice the difference between the default device profile as opposed to the "communication" profile? The same is true when you install a bluetooth speaker on a PC: typically you'll see both device profiles, with this type of disparity.

Now here's the thing: if you have root on your unit, you can technically specify which audio band/stream you want to have any particular audio stream broadcasted to. I don't believe it's available on the frontend, but if you use ADB or SetEdit to view the settings db tables (can't remember if it's Secure Table or Global Table), you can actually see all the individual configurable keys and current values for each type of volume (e.g., Call Volume, Guidance Volume, Confirmation Volume, System Volume, etc etc), which audio streams get suppressed when you mute, and more. I've not done any testing myself, but I'm almost certain somewhere in here there should be a key/value pair that can alter the subject behavior.

Another place to look at would be the Developer Options UI. There is an option to select the BT stack in use by the device, as well as some options regarding the transmission bitrates, which have an effect on the audio quality. There is also some BT version formfactors that are defaulted to older versions (I assume for interoperability bewteen devices).

Finally, there is typically an option available on the frontend under the BT device settings where you can select the codec in use (if available). It's typically defaulted to one that isn't HD (AAC is the HD codec, I want to say off the top of my head).

Anyway, I'm pretty sure between these places, a solution can be found? But like I said, I'm not familiar with AV stuff, so hopefully someone else with technical knowledge on the subject can pitch in.