Indeed I answered but it's like my message has lost itself on the deepness of the web limbs, so I will write it again.
I received the new RD pro will unboxe it tomorrow, but I don't think it is possible to play the audio recorded message before the beeps or/and flashes.
Non the less I think it is possible manage to do it by using the TWC mode (Two Way Communication).
Contrary to the normal mode, the TWC will send 6 or 7 series of clues, with 30 seconds between each one, and an increasing intensity of the flashes/beeps/recorded audio messages.
So if I record 15 seconds of silence before saying my vocal message, since each set of flashes and message is repeated every 30 seconds, the 15 seconds delay will do what you suggest: a flash or/and beep will be preceded by the vocal message.
I can record a message telling to myself to be aware of the next flash/beep serie.

The second advantage of the TWC mode is that I can stop the mask from the dream world by doing a CODE with my eyes, so if I am lucid during the process, I can stop the clues and prevent the mask from waking me up.

When I am not trying to use my RD to have a WILD/DEILD, I can use it to test your technique.