Hello @ccheng,
I just answered a similar question for another user. Please see below:
While in the meeting, please click the up arrow next to your Mute/UnMute icon in the bottom left corner of your Zoom client. This will allow you to verify which mic is active. It sounds like Zoom may not be set to the mic you want to use and may be using the built-in computer mic instead. Select the proper mic from the list and see if this resolves your problem.
If this resolves your issues, please click the Accept as Solution button on this thread so that others can benefit from this as well.
Thanks!