2 audio sources won't play at the same time

Doh. My bad, I left a deprecated sound management script on the enemy. Removed it and all is well.

Do something like.

var audioClips : AudioClip[];

//to call each audio clip
audio.PlayOneShot(audioClips[0]);

Also if you are using the Audio for GUI elements then disable 3D Sound in the inspector for that audio file.