Doug Brown writes:

Recently, Aidan Halpin, a reader of this site, asked me if I could do the same kind of startup sound customization on his iMac. This particular iMac is officially known as the “iMac (Slot Loading)” and has a model identifier of PowerMac2,1.

He sent me Apple’s last firmware update for this model: iMac Firmware Update 4.1.9. I went to work looking at the update contents to see if I could figure out how to modify the chime the same way I did with my Power Mac G3. I thought it would be fun to take everyone along for a ride and show exactly what was involved in changing the sound. And of course, this post wouldn’t be complete without also sharing the code for the utility I created to inject the new chime into the firmware update file.

So when Aidan asked if I could look into changing the sound on the iMac, I took the compressed sound data I had found 10 years earlier in my Power Mac’s ROM and searched for the same chunk of data in the iMac’s firmware update file. Sure enough, the sound was the exact same compressed sound, right down to every byte.

See the video below and more on the methodology here.