The 7-inch HD van/caravan monitor is a good choice for you. The solution to this problem is simple and not that complicated.
The reason why there is no sound after using the HD interface to connect the monitor is that the HD interface contains audio + video signals. When using the computer's HD interface, the system outputs audio signals from the HD device by default. At this time, if the HD device does not have a sound playback function (for example, the author's HD LCD monitor does not have speakers), the computer will not make any sound!
After checking later, I found that there are two devices under the "Sound, Video and Game Controllers" column in the Device Manager, namely "AMD High Definition Audio". Intel or Nvidia, etc.) and "High Definition Audio.
We only need to disable AMD High Definition Audio (the front of your computer may not be AMD, this will vary depending on the display chip your computer is equipped with), and then the computer can connect to the HD monitor. There were no sound issues after that.