We resolved the issue, so far, by deleting (and not reinstalling) the Realtek Driver and using the Windows Sound Driver. We also disabled all other devices in the Control Panel Sound. No muffled sound and the audio keeps up all the time.
Hey everyone, thanks for reading my thread. The Laptop series Latitude 5520 has a lot of audio problems in my company. We bought a couple of them and all have audio problems with no exception. We noticed this problems in programs like Teams, or SwyxIt with different headset types and models: The microfone/playback sound is sometimes not working at all (others can't hear you, or you can't hear others.) and sometimes the microfone has a muffled sound or a sound of really bad quality. We tested the following headsets: We tested in following programs: Example of devices with which we have no problems at all, regardless of the headset or program: We already tried the following: We already contacted Poly they couldn't help us out. We also contacted Dell directly, no response to the open ticket since now (ticket was "escalated" to the 3rd level). I would appreciate any kind of input/soltution. Thanks in advance! 
Latitude 5520 muffled/no audio with headsets in programs like Microsoft Teams, 
or SwyxIt