What is the spelling of yawning sound?

There isn't one specific spelling for the sound of yawning. It's usually represented by words like:

* Yawning

* Yawn

* Aaah (or Ahhh)

* Sigh (if the yawn is quiet or accompanied by a sigh)

* Gaping (if the yawn is wide and exaggerated)

You can also use onomatopoeia to imitate the sound:

* Ahh-hhh

* Yaaawn

Ultimately, the best way to represent the sound of a yawn depends on the context and the desired effect.