I want to change the alarm sound.
The alarm is a beeper with a power activated by a simple transistor. Remove the connection to the beeper and insert the sounding device of your choosing. Or feed the signal to a dedicated English hi./.lo police alarm if you like. The US military alarm for disinterested remote geographical ugly lone zone places that cannot be ignored is a baby's wailing scream cry. But you may want Nero's tune, to induce one to make fiddling Love while Rome burned ;-)
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Then you will need to splice in to where the controller on the detector signals the alarm and have that trigger the sound you want. Best way to go about that may be to use a micro controller like the Arduino and the an Arduino Wave Shield so it will play the MP3 file of your choice. But in doing so you render the alarm useless in the eyes of the law and your home owners/renters insurance. If a fire occurs and someone dies you will be held liable when they find a modified smoke detector could be to blame for everyone not getting out. So proceed at your own risk. From what i can tell the second instructable listed on the right hand side of this page has some helpful info about smoke detectors.