1. Download Z4Root from the app market and root your phone.
2. Once rooted, download Root Explorer from the app market.
3. Open Root explorer and navigate to system>media>and then rename the droid.ogg file to droid.old (also at the top make sure you click "mount R/W" or you won't be able to change the name)
And that's it! You'll have yourself a nice quite startup the next time you reboot your Droid Pro. Enjoy!