Hi there, lexjohnston!
Thanks for reaching out to the Apple Support Communities! It sounds like your music is getting quiet when connected to Bluetooth. If this is happening and you're not receiving a notification when it happens, check out this article for How to connect your iPhone, iPad, or iPod touch to your car. Specifically, the steps here:
"Connect using Bluetooth
- Restart your device.
- Go to Settings > Bluetooth, and turn off Bluetooth. Wait for about 5 seconds, then turn Bluetooth back on.
- Check the manual that came with your car for more information on how to pair with a Bluetooth device. Most cars require a phone setup on the car display.
- If you've connected multiple phones to your car stereo, try renaming your device: Go to Settings > General > About > Name, and type a new name. Then try connecting again.
- Update your device.
- Make sure that your CarPlay stereo is using the latest firmware from the car manufacturer. Check your car's manual for more information.
If you still can't connect, contact Apple Support."
Even though this article is specifically for issues connecting, the steps would be helpful for situations with the connection not working properly as well.
Have a wonderful day!