Volume level intermittently drops I've noticed issues where the audio level intermittently drops.

When playing audio the volume will intermittently drop off (doesn't stop just gets quieter) then after some period of time it will return to normal volume.

This happens consistently when using CarPlay (plugged in via lightning connector) or on car Bluetooth.  

The same issue happens on Bluetooth headphones.  This issue didn't appear until after iOS 11, but has been going on ever since then despite several updates since then.

This issue first appeared on my iPhone 7 after the iOS 11 updates.  Now I've just upgraded to an iPhone 8 Plus and it is having the same issues straight out of the box.

This is a problem across the phone itself, but for some reason is more noticeable in some situations than others.

For example; this issue occurs with Apple's "Music" app, but the volume change isn't as drastic as with the "Amazon Music" app.

When plugged in with a pair of lightning headphones this issue doesn't happen, but when using bluetooth or carplay  (even using usb/lightning adapter) it constantly drops in and out.
Are you receiving texts, notification, etc when the volume drops? Most bluetooth interfaces will dim the volume when a notification arrives, even if the notification sound is set very low. Check your notification setting on the bluetooth device by adjusting the volume without any music playing. 

Try using your phone in silent mode and see if the problem goes away.