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
[Audio] UnfocusedVolumeMultiplier=1.0
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.
Doesn't that work anymore?