When I playback my videos on the computer, the sound sometimes is lagged or simply there is no sound. What happened? And why I have trouble playback the videos sometimes?(YI 4K+ Action Camera)

Using high resolution or high frame rate to shoot videos, such as 4K/60fps may result in playback errors when viewing on a computer or TV.
You may need a higher performance computer for video playback. Or try downloading a third-party video player with a more advanced decoder.