In general, this is something to be implemented in the game's code depending on the desired outcome when the application is in the background.
The relevant general documentation can be found here: https://www.audiokinetic.com/en/library/edge/?source=SDK&id=workingwithsdks_system_calls.html#system_calls_background
In this case, if the game is allowed to keep sending events to Wwise (its game loop still runs), AK::SoundEngine::Suspend(true) is the call that should be made to allow the processing of those events, while still muting the sound.
This is activated in the Unity integration through the "RenderDuringFocusLoss" platform setting.
Instead, if on focus loss the game loop is paused, then no events can be sent to Wwise and AK::SoundEngine::Suspend(false) should be called, so that all Wwise events processing is stopped. RenderDuringFocusLoss will achieve this outcome.
For reference, the Unity integration code responsible for this handling in is located in Assets/Wwise/MonoBehavior/Runtime/AkSoundEngineController.cs.