I was using "Q" a lot in FSX and P3D to mute the sim when I needed to and I'm desperately looking for this function in MSFS.

According to this thead https://forums.flightsimulator.com/t/add-toggle-mute-unmute-sound-shortcut-key-during-flight/294577 there's still no key binding to mute MSFS?

 

Am I missing something or isnt there still no key to mute the sim?

Personally, I downloaded EarTrumpet (A much improved version of Windows' sound flyout bar, to control volume / mute on a per-app basis). That has an "Actions" interface with basic triggers, then you can specify a hotkey to perform action, like muting a specific application, i.e MSFS. Not sure if "Actions" is available in the regular build but it's definitely in the Dev build. 

Something you may consider if you don't find another way. 

I´ve posted a solution on the official MSFS Forum, especially for online flyers (VATSIM & IVAO):
My link to the MSFS forum: 

Hey guys!

I´ve found a solution for the MSFS key to mute / unmute topic.
Sorry for my english, it´s not my native language.

There is a freeware program called “Sound Volume View 2”
Scroll down for the download link.

Step by step guide:

1. After downloading the the app, open Sound Volume View and be sure MSFS is running in the background.

2. Right click on Flightsimulator.exe and select Create Desktop Shortcut → Mute / Unmute Switch
   Confirm with Yes

3. Now you find a shortcut on your desktop Disable Switch FlightSimulator.exe

First solution:
Now you can choose with right click on Properties if you wanna send this to your taskbar. If you click on it, the MSFS Sound is instant muted, click again and the sound of MSFS is unmuted.

Second solution:
If you wish you can choose under Properties a shortcut with a keybinding you want.
But keep in mind, this option take up to 2 seconds, when you pressing your desired keybinding for muting and unmuting.

The Sound Volume View App don´t has to be running in the background!
Just setting it up one time and it´s done.

Have fun!