audio level of sustained vocal note is reduced during recording

I’m using Audacity ver. 2.3.2.
My OS is Windows 7.

When I record dialogue, and sustain the vowel of a word, the audio level drops after about a second of the initial tone. How can I prevent this from happening?

Windows audio enhancement is the culprit, (e.g. echo-reduction, noise-reduction, etc).
Learn how to turn of Windows audio enhancement here …