Actually, MattyP is spot on. Motorola reduced the volume on the OGG file for the boot sound because apparently it was complained about enough that they felt it was necessary.
If you are rooted, you can
[*]Download
droidhd.ogg (an amplified OGG file of the boot animation sound)
[*]Then using a root enabled file manager, such as Root Explorer, go to the folder where the file was stored (/sdcard/download).
Note: If you want to retain the quieter version, first rename this downloaded file to droidhd.og2.
[*]Then select the file and move or copy it to the root folder /system/media.
[*]Next, make sure you are in the mode which allows writing to the folder. In Root Explorer you press on the top right corner of the screen where it says "Mount R/O", and change to "Mount R/W".
[*]Now paste the droidhd.ogg (or .og2) to the folder.
Note: If you are interested in preserving the original quieter version, find the file droidhd.ogg in that folder and rename it droidhd.og1, then rename droidhd.og2 to droidhd.ogg.
[*]Finally, change the file permissions for the droidhd.ogg file to rw-r--r--
[*]Reboot and enjoy.
This information and the link for the file came from another forum, though I am not totally sure I can link to it so I copied, cleared up the instructions and posted here for your benefit.