Huawei's decision to output audio through the USB port means that sound quality will degrade when a USB cable is connected. We measure exactly how much worse and softer the audio becomes with a sound meter and microphone.
https://www.notebookcheck.net/Connecting-a-USB-cable-to-the-Mate-20-Pro-will-reduce-volume-by-about-1-dB-A.363011.0.html
Saying it's a decibel quiter is not a good representation as decibel is an exponentially increasing scale eg the difference between 5-4 dB and 100-101 dB is veru different