Is it 'incoming' audio that's the problem or 'outgoing' audio. If it's th former, the problem probably lies with the other person's computer/tablet/phone. If it's the latter, it's probably at your end.
Whoever actually has the problem needs to check that their computer/phone/tablet hasn't lost the setting which tells it to allow Zoom to use the microphone on that device.
If the relevant device is an Android one (such as the majority of mobile phones) or an iOS one (such as an iPhone or iPad) see the stuff about 'Permissions' for the microphone here:
https://support.zoom.us/hc/en-us/articles/204484835-My-Audio-is-Not-Working-on-iOS-or-Android
If the device is a Windows 10 PC or laptop, use the Windows search box to navigate to Microphone privacy settings. Switch on the 'Allow apps to access your microphone' toggle, and enable access for Zoom.