Please make sure your laptop/ cellphone/ tablet  supports thunderbolt 3 or Display Output (DP ALT mode), OTHERWISE IT IS NOT COMPATIBLE. Please check with the device manufacturer.

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

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.