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This should do the trick,but.........
It should be noted that this circuit does not "latch". The relay and LED operate momentarily in response to audio peaks.
Never the less it's a good place to start.
Yeah, I've always wanted to do that too. Is it so you can have your own alarm circuit? Just wondering. And yes, it would pick up ambient noise. So, the only way to do this, is to take your alarm clock apart, and try to trace the circuit back from the speaker to the "alarm out", and solder your circuit there. Sorry it's not much in detail. Let me know what you find out and good luck.