Audio AudioSource.Play() lags the second time it is called

I have a button that calls a script on click to enable (load) a menu.

The menu loads in about half a second.

The first line of the script is AudioSource1.Play();

The first time I click the button, the audio plays immediately with no delay (before the menu is visible).

Every time after that, the audio clip plays after the menu is loaded (visible).

I can solve this by replacing the line of code with AudioSource1.PlayOneShot(AudioSource1.clip);

The audio file is uncompressed .WAV with a half second duration. It is preloaded, (I've tried tweaking every other setting as well).

Any ideas on what is causing this? It seems like an issue with the Unity Engine.