Just connected an ASUS monitor to my MacbookAir. It mirrors perfectly, but when I play any videos, they go into super fast forward with distorted sound.

Hello Grynous,

Thank you for using Apple Support Communities. We’d like to help out and recommend some steps to see if we can get this issue sorted out. Let's try the steps below:

1.) Shut down your MacBook Air.

2.) Disconnect accessories from your MacBook Air. Also, connect your external monitor to another port.

3.) Turn on your MacBook Air and test to see if this issue continues to occur.

If the issue continues to occur, try testing this issue in safe mode. This step can help isolate possible software related issues: How to use safe mode on your Mac - Apple Support

Apple silicon

1.) Shut down your Mac.

2.) Turn on your Mac and continue to press and hold the power button until you see the startup options window.

3.) Select your startup disk, then press and hold the Shift key while clicking “Continue in Safe Mode.”

4.) Log in to your Mac. You might be asked to log in again.

Depending on your results, follow the steps under “If the issue continues in safe mode” or “If the issue doesn't continue in safe mode”.

Also, if you have access to another monitor, try using that monitor. See if you're able to recreate this issue with video playback. This is another step to further isolate this issue and confirm if it occurs with only one specific monitor.

Feel free to keep us posted. Thank you for using Apple Support Communities. Cheers.