8K USB-C - デュアル DisplayPort 1.4 アダプター (MST デュアル モニター付き)

MST is a technology that allows multiple external monitors (with different graphic contents) to be connected via a single USB-C connector, operating in minimum three individual screens.

Mac computers do not support dual extended displays through Thunderbolt 3 port, therefore this adapter is not able to support a second display.  The new M1-based Mac mini computers are exception because it has an additional HDMI port, allowing users to connect two additional display in extended mode via two separate ports.

Please look for adapters or hubs with DisplayLink technology. It can bypass the limitations of the single display support in Mac computers. We have a docking station (model number D001A) that support DisplayLink technology. Details please visit https://www.unitek-products.com/products/uhub-s7-7-in-1-universal-docking-station

An MST USB hub is not able to increase the maximum number of displays that a display card supports. 
For example, if your display card only supports up to three displays, the display limit will still be three displays even if you are using an MST hub. 
Besides, your display card has to be compatible with MST technology in order to apply Extend Mode to your external displays.
We recommend you to check your display card to make sure that it can support MST as well as the number of displays that you are using in your setup.

If you are experiencing flickering issues with the MST USB hub, your displays may be at a 59Hz refresh rate. If it does, you have to adjust the refresh rate to 60Hz. 
To do so, go through the following:
1. Right-click anywhere on your desktop.
2. Click Screen resolution or Display settings.
3. At the top of the screen, select the monitor that you want to change the refresh rate for.
4. Click Advanced settings or Display adapter properties.
5. On the Monitor tab, click the Screen refresh rate drop-down list.
Note: The above steps may vary depending on your operating system and graphics card.
 

To set up completely, you have to adjust setting in 'Displays' and 'Audio'.

To set the display:
1.With your display monitor connected to your Mac, open Apple -> System Preferences.
2.Select 'Displays' with its big blue screen icon
3. Go under 'Arrangement' section
4. Check the Mirror Displays checkbox

To set audio:
1.With your display monitor connected to your Mac, open Apple -> System Preferences.
2. Select 'Sound' with a speaker icon
3. Go under 'Select a device for sound output' section to see if you see your display monitor as an output
4. Choose the monitor instead of built-in speaker as your audio outlet

To set up completely, you have to adjust setting in 'Screen' and 'Audio'.
To set screen:
1. Right-click any empty area of your desktop, and then click Screen resolution
2. Click the multiple displays drop-down list, and then select 'Duplicate these displays'.
3. If the display monitor is not listed, click 'Detect'.

To set audio:
1. Open Control Panel
2. Select 'Hardware and Sound' - 'Sound'
3. Under the 'Playback' tab, select the proper device as default device. 
4. Click "OK". Justify the volume at the down-right corner if needed.

There could be a couple reasons. Check your audio settings on your MacBook. Make sure the correct audio is selected. 
Step1: Go into System Preferences
Step2: Click Sound
Step3: Select the HDMI source as opposed to the Internal Speakers