I saw that someone had asked for the android_audio.mp3 file in a lower volume version. It was driving me nuts too, so I just played with it and figured it out. I have attached it to this post as a zipped file. Unzip it to get to the android_audio.mp3 file.
Instructions for switching over to this new file if you desire to:
1. Connect your Incredible to your PC
2. Mount it as a hard drive in debugging mode
3. Unzip the file attached to this post
4. Go to wherever you saved your android_audio.mp3 original file on your computer and delete it. Put the new file in its place.
5. Open cmd prompt in windows
6. Do the cd command and set it to wherever you put the android_audio.mp3 file
7. Type:
adb shell rm -r /data/local/android_audio.mp3 (enter) <---- you may or may not see anything after you do this, but it worked. If you want to be sure reboot the device at this point and check. You should have no audio with your boot screen. Once it is back on remount it as a hard drive (it should still be in debugging mode). Continue onto step 8.
8. Type:
adb push android_audio.mp3 /data/local (enter)
9. Reboot the device and the audio playback with the boot screen should be lower.