MacOS – “Hey Siri” suddenly stopped working on MacOS Mojave

Siri works perfectly after I manually click on the Siri icon on my TouchBar or click on the menu bar Siri icon so there is no issue with my microphone.
Also there are no user accounts to manage in my Parental Controls Preferences.

However it does not start when I speak "Hey, Siri".

I have tried the following:

1. Unchecking and checking listen for "Hey, Siri"
2. Reset Ask Siri in System Preferences
3. Restart my Mac
4. Reset NVRAM
5. Reinstalling the OS

Nothing has worked. Please help me fix the issue.

Update 1:
In Safe Mode even Siri does not work let alone Hey Siri. It gives me an error stating "No microphone found."

Update 2:
After taking advice from a Senior Apple advisor, I formatted my hard disk and reinstalled macOS. The problem was fixed but not for long. Today I vividly remember that this problem started after pressing the Siri icon on my Touch Bar while being connected to my bluetooth headphones

Version: 10.14.6 (18G95)

Model: MacBook Pro (13-inch, 2019, Two Thunderbolt 3 ports)

P.S. Other problems that might be related:

1. The familiar Siri feedback sound that plays once Siri starts also has stopped working when I start the app manually. It however works when I am connected to my headphones.
2. The volume feedback beep that occurs when you change volume using the Touch Bar also has stopped.