If you can't get that "success" message to work, follow these instructions:
1. First of all download HDAPM from mckinlay.net.nz/hdapm
2. If Safari doesnt open it, click on the .DMG file yourself. You see 3 files: a readme, the hdapm.plist and the program itself.
3. Now its time to reveal the hidden folders in Leopard. Go to the Terminal (you can search for it in spotlight; this is the searchbar with the magnifying glass at the top-right of you screen). Copy/paste the following line in the Terminal and press enter:
defaults write com.apple.finder AppleShowAllFiles -boolean true;killall Finder
Leave the Terminal open for now.
4. Open the Finder, go to your Macintosh HD and then to the map /usr → /local → /bin. If there is no map /bin you will have to create it. Do this by Copying and pasting the following line in the Terminal and press enter:
sudo mkdir /usr/local/bin
Ignore the warning and give your admin password. The map /bin has now been created.
5. Copy/past the program hdamp (not the .plist) to the /bin map.
6. Go back to your HD and look for the map /library and then /launchDaemons. Put the hdapm.plist in here and confirm it again with your admin password.
7. Restart your Mac. After reboot, open the console (use spotlight again) and search for the string hdapm (searchbar is again on the right).
8. You will find:
Setting APM level to 0xfe
Success
If this appears, you are done. Congratulations!
But . If you find:
localhost com.apple.launchctl.System[2]: launchctl: Dubious ownership on file (skipping): /Library/LaunchDaemons/hdapm.plist*
you need to go on to point 9.
9. To tackle this problem you have to download the program Lingon from lingon.sourceforge.net. Open it.
10. You have to create a new .plist for hdapm. Click on New (top-left in the program) and check User Daemons and then create. In the 1st field fill in:
hdamp2
the 2nd field:
/usr/local/bin/hdapm disk0 max
and following the 3, check:
"Run it when it is loaded by the system (at startup or login)"
You created hdamp2, as you can see on the left. Click on hdamp and disable it (uncheck the box with enabled) or delete it from /HD/library/LaunchDaemons. Reboot you Mac.
11. If everything went right, your blip/pop sounds are gone. The only thing left to do is hide your folders again. Go to the terminal and copy/paste the following line and press enter:
defaults write com.apple.finder AppleShowAllFiles -boolean false;killall Finder
The End!
Sorry, I know it is an old thread, but after following your guide I've got this message on the console:

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there is the error message about file ownership, but the following message report a success in setting APM to 0xfe.
Do you think hdapm is correctly set in my MBP ?
Thank you