I have this issue! It started about a week ago. At first, I thought it was an issue with the CarlinKit wireless adapter I use to connect CarPlay wirelessly. But removing that and connecting by USB cable didn't solve the problem. Here's what I'm experiencing...
The first symptom I noticed was the radio volume would jump between loud (16) and soft (8). Then it would be fine. Then it would jump back and forth again. I tried everything. The issue is present on FM, SiriusXM, and anything streaming from my iPhone. I restarted my iPhone. I deleted the HondaLink app, which I had just installed a few days before the issue began. I've tried different USB cables. The only thing I haven't tried is someone else's iPhone.
I've been able to replicate the issue, but only at certain times. I think I've determined it's linked to the volume while using the phone. While listening to SiriusXM (through the car, not through CarPlay), with my iPhone connected by USB, the volume was fine. When I opened the Phone app in CarPlay, the volume muted, and wouldn't go above 8. Closing the Phone app, the volume returned to normal. I was able to repeat this several times. Then, this morning, I wasn't. Opening the Phone app had no effect on the volume (meaning I had full range).
It seems to make the most sense that this issue is with the latest iOS update 17.4.1. I don't think anything else in the loop has changed. No recent updates to my car system. Stuff like this drives me insane (no pun intended). Troubleshooting it is a nightmare!