Use the Macintosh speech synthesizer to speak a message.
speak (or say) “the message to speak”
Both verbs also allow adding the optional in (number, seconds to schedule the speech for) parameter.
say “this is delayed fast quiet speech in an Irish accent.” voice “moira” rate “220” volume 0.5 in 5
speech is asynchronous, the script continues immediately and the speech continues in the background. If many phrases to speak overlap they are queued up and spoken one after another. If you accidentally queue up a large number of phrases you can use the Stop Speaking verb to clear the queue.
There is a bug in OSX 10.11 (El Capitan) that keeps airplay sound outputs form showing up or being used as outputs for sound or speech in any application. If you wish to send sound down airplay outputs have a look at scripting “airfoil” to control which outputs it goes down. This bug remains in OSX as of 10.11.4 and has not been fixed by Apple.