All circuits will have some sort of latency. Not to mention the QRN and maybe QRM that could trigger your circuit unnecessarily.
So instead of designing the elusive "zero-latency with complete noise immunity" circuit here is an alternative:
Do it in software. Keep the audio path open all the time. With the right program it would be less likely to miss any part of a real preamble.
These kinds of input circuits are rarely connected to speakers so it wouldn't bother you by the white noise in between.
Doing it this way could ensure you didn't miss the preamble because of latency in an external circuit.
Of course, be sure to do your due diligence and post this same question directly into Google and see if/whom has done this before you.