Hi weiqi163,
Welcome to Apple Support Communities. We understand that you're experiencing intermittent issues with the sound and haptics for your Apple Watch Series 7.
First, to begin isolating the issue, we'd recommend ensuring that both your Apple Watch and companion iPhone are running the latest OS versions.
Update your Apple Watch
Update your iPhone, iPad, or iPod touch
You'll also want to check the speaker of the Apple Watch for any debris, along with removing its case (if applicable) to ensure it's not covering the speaker. If you find debris, you'll want to gently clear them away. How to clean your Apple Watch
Turning the volume up, along with managing the haptic alerts on the Apple Watch can also help: Change the audio and notification settings on your Apple Watch
Let us know if you continue to experience issues after completing these steps.
Regards.