Game audio stops when you alt-tab?

The music cuts out the instant you click your other monitor. That is behavior the game's engine turns on by default, and it can usually be turned off.

Why it happens

Game engines mute on focus loss on purpose, so a game left in the background does not keep playing at full volume. Source games do it with a console variable. Unreal games drop a volume multiplier to zero while the window is inactive. Unity games either keep running in the background or stop entirely, depending on a choice the developer made when building the game.

Free fixes first

Look in the game's audio settings for "Mute when unfocused", "Mute in background" or "Background audio".
Source and Source 2 games: open the console and enter snd_mute_losefocus 0, or put that line in the game's autoexec.cfg so it sticks.
Unreal Engine games: open Engine.ini in the game's folder under %LOCALAPPDATA%, inside Saved\Config\Windows (or WindowsNoEditor in older games), and add:
```
[Audio]
UnfocusedVolumeMultiplier=1.0
```
The folder is named after the game's internal project name, which is not always its title.

When the free fixes are not enough

Many games have no audio option, the right config file is hard to find and differs by engine, and some games put their settings back after an update. Unity games cannot be changed this way at all, because the setting is compiled into the game.

How Second Screen Gaming fixes it

For games it recognizes, it writes the right setting into the game's own configuration: the Source console variable, the Unreal audio values, or a rule written for that specific game. Every file is backed up before the first edit, and turning the fix off puts back each setting it changed.

For other games, the focus assist answers the focus change the way a real return to the game would, which keeps engines such as Unity playing their sound.

If the game runs on another monitor, Send sound to that screen can play its audio through that monitor's own speakers.