My problem solved with the following instuction:
1. Download Skype 18.104.22.168 (only this version)
2. Uninstall the recent version
3. Install the previously downloaded Skype 22.214.171.124
Hope Microsoft will fix it with a software update. It seems not only Dell customers are affected.
I have followed your advice. Uninstalled Skype version 7 and used Revo Uninstaller to ensure all the registry entries were removed. Then found the version 126.96.36.199 on File Hippo and downloaded it. Then re-enabled my Microphone, then installed the version 6.
Regrettably, it still does not work either with Skype or Cortana. You can just about hear a very muffled playback - not sufficient to understand the words, with the volume turned up to the point where feedback starts.
So still waiting for a fix. To late to return to Windows 8.1 on my laptop as the 30 days have expired.