Hello Sander148,
Go to settings and scroll all the way down to Webex Meet in the left panel and see if the microphone switch is turned off . If it is, turn it on.
If this does not solve the issue then I would first restart the iOS device by pressing and holding down the power and the up volume buttons until you get to slider to fully turn it off, wait a minute, and turn it on again.
If this does not work then my next option would be to delete the app and reinstall it.
Hope one of these options fixes your issue.