The Play Sound action plays a selected sound through a specified output device at a selected volume.
The Play sound pop-up lists all the installed system sounds, but the other choice allows you to choose any other saved sound.
The out device pop-up lets you choose from any connected output device.
Use the with volume slider to select the sound level.
The action will wait for the sound to complete before continuing unless you turn on Asynchronously (v8+) in the gear ⚙ menu (or unless it is the system beep which always plays asynchronously).
Note that the System Beep sound has not controls, so it always plays through the default sound effects output device at the default volume asynchronously.