I'm having the same issue with my 7390 (XPS 13). I've re-installed everything.. etc etc. All other VOIP programs work well (Slack, etc.) but when using Zoom my mic fades in and out. Please help Dell!
Same problem, same machine I just bought. Dell has told me it's a software issue and they can't help unless I pay for their software support. Zoom has told me their engineers are working on this as a known issue. I expect there to be some driver update that comes along and fixes it eventually. Until then I think we're stuck, either using phone audio, the zoom web client (mine works with that!) or finding another workaround, eg using another device to zoom.
I just bought this laptop in August and whenever I try to zoom or have a video call on this laptop- the microphone doesn't work well. It fades away as soon as I finish my sentence. I am a university student and will be needing this laptop for the coming three years.