Hi!
As I currently understand, audio playback is not possible with only the AudioServer. Similar to the way an AnimationTree requires an AnimationPlayer node to be present to actually play animations with the AnimationTree, an AudioServer requires a node to handle the actual audio playback— such as an AudioStreamPlayer. I am not exactly sure what tangible effect you are trying to achieve, but rest assured, an AudioServer can certainly work hand in hand with an AudioStreamPlayer.
I hope this response serves you well.
Maxwell