Hello Hout007,
We would like to see how we can help you with your videos not playing as expected. Are you having this issue with all apps, or only one specific app? Check out the steps in this link and get back to us with your results: If the internal speakers on your Mac aren't working - Apple Support
"Check your sound settings
First, disconnect any cables from the USB, Thunderbolt, or 3.5mm audio ports on your Mac.
- From the Apple menu , choose System Settings (or System Preferences).
- Click Sound.
- Click the Output tab, then select the built-in speakers. If you don’t see built-in speakers as an option in the Output window, contact Apple Support.
- If the Output volume slider is set low, move the slider to the right and test again.
- If the Mute checkbox is selected, deselect it and test again.
- If you still don’t hear any sound, contact Apple Support."
Cheers!