Enabling sound in the background is an existing feature in Unreal Engine, but it has not yet been exposed to us through the in-game settings by the devs. I'm sure it'll come eventually, but in the meantime I looked for a way to enable this so I can hear my teammates talking even if I have to alt-tab out of the game for a few seconds. Here's how you can change your settings files to enable it:
Close Hell Let Loose if it's running
Open the file explorer
Enter the following path in the top: %USERPROFILE%\AppData\Local\HLL\Saved\Config\WindowsNoEditor
Open Engine.ini with notepad or any other text editor
Add the following two lines at the bottom of the file
The value can be changed from 1.0 to any decimal value between 0 and 1 if you want the sound to play at a reduced volume. For example, 0.5 will make the volume 50% when you alt-tab out of the game.