Allow sound.play to pass index of sound, by default play first sound

[Mal_Duffin]

@Mal_Duffin

have entity.sound.play() use by default the first sound slot, if it exists.

I’m just flagging this again as a useful and sensible default bahaviour.

If someone does sound.play(), detect that the first parameter is undefined and just play the first sound attached - with a check to see if there is a sound there of course - as this is very likely what a new, and experienced, dev would want most of the time.