Per a few member's requests, I am putting this HOW TO together. It's quite simple, and really should work for just about any Gingerbread ROM for the Thunderbolt out there right now. There may be some small variations from one ROM to another, but the basics should all be the same. Hopefully I explain it thoroughly enough that you can figure it out if there happens to be some small differences.
First you need to be aware of where the bootanimation is to be found. I have looked through a couple different ROMs, and found them in a couple different places. Since this is in the Gingeritis 3D ROM section, I'll base this how to on that ROM.
Look in /system/customize/resource/
(Note: In other ROMs, you might find the bootanimation in /system/media/)
You should find the following file in there: VZW_bootanimation.zip
That is the bootanimation for this ROM. You can change that to whatever bootanimation you'd like, and there are several sources for custom bootanimations available to you. How you decide to go about choosing your custom bootanimation is entirely up to you.
Next, you need to decide what sound/song/music it is that you want to play to your bootanimation. Remember that bootanimations don't last very long. You can make your audio clip as long as you'd like, but once the phone is done booting, it will simply stop playing your boot sound and go to the lock screen. So having a really long sound clip is simply going to waste space. The only time you might make it through a sound clip longer than 15 or so seconds is if you wiped your dalvic cache. The phone will rebuild the dalvic cache during the first boot, and as such, you'll be able to enjoy your sound clip for as long as it is booting (ie. the sound will play for as long as you made it last, or as long as it takes the phone to rebuild the dalvic cache and complete the boot cycle. Whichever comes first.).
Once you have your sound clip picked out and trimmed to your liking (make sure it is saved as an mp3), you need to give it a name. For simplicity sakes, we'll name it the same as the name of what the stock boot sound would be, which is: Verizon_Thunderbolt_3sec.mp3
After giving it its name, you need to move it to the proper folder within the phone's system. The location is: /system/customize/resource/ (that should sound familiar, as it's the same folder where the bootanimation.zip is located)
OK. Now that you have your custom bootanimation.zip and custom boot sound file in the proper location, you will now likely need to make a small change to an XML file that is found in the following location: /system/customize/CID/
The file you're looking for is called: default.xml
Assuming you've used Root Explorer to access these folder and file locations on your phone, long press the "default.xml" file, and when given the option list, scroll down and select "Open in Text Editor".
Once you've done that, scroll down to the bottom of the file and find the following line:
<BootAnimation animation="/system/customize/resource/VZW_bootanimation.zip" audio="/system/customize/resource/Verizon_Thunderbolt_3sec.mp3" audiostart="VZW" />
This is telling you how the system is accessing/referencing both the bootanimation and the boot sound (both by location and by name). Notice that it gives you the full string to the location, and the exact name by which it is called.
(For those of you who might be performing this modifcation on a different ROM that might have the bootanimation located in a different folder, this is how you figure out where to go to find and where to put the custom bootanimation and boot sound. Inversely, you could also alter the string and change where and by what name the system will access/reference the bootanimation and sound clip. Then you just have to make sure to place the animation and sound clip to the location you specify in the string, and make sure the names match.)
It is also telling you when it will start playing the audio clip where it says: audiostart="VZW" />. In this example, it is telling you it will begin the boot sound as the VZW folder within the bootanimation.zip is accessed by the system to display the images inside that folder. The stock/Verizon bootanimation has only one folder inside of it, and it is named VZW.
Most of the custom bootanimations I've looked over consist of 2 folders within the archive. Part0 and Part1, where Part 0 contains the initial images of the bootanimation, and Part1 contains the images that will repeatedly play in a loop during a boot cycle. As such, if you do not edit the string above where it calls out VZW as the folder that activates the boot sound, you will not get any sound during your boot cycle. You need to delete VZW, and add the name of the folder that contains the images that you wish to have sound played to. You may want the sound to start just as soon as your bootanimation begins to play. If that's the case, and your bootanimation archive is configured with a Part0 and Part1 set of folders, simply change VZW to Part0. If you would prefer that the sound didn't start playing til you begin the second set of images that will play in a continuous loop, then change VZW to Part1 instead.
Now, the default.xml should read like the following:
<BootAnimation animation="/system/customize/resource/VZW_bootanimation.zip" audio="/system/customize/resource/Verizon_Thunderbolt_3sec.mp3" audiostart="Part0" /> (for if you want it to start playing as soon as the bootanimation begins)
<BootAnimation animation="/system/customize/resource/VZW_bootanimation.zip" audio="/system/customize/resource/Verizon_Thunderbolt_3sec.mp3" audiostart="Part1" /> (for if you want it to start playing after the initial bootanimation images, or in other words, only during the continuous loop part of the bootanimation)
If you aren't sure how your bootanimation.zip is configured, simply view the custom bootanimation archive either on your computer or on your phone using a file explorer. Within the archive, you will see how the folders are named.
If your custom bootanimation is similar to most of the ones I've looked at, you will find Part0 and Part1 inside of the archive. Obviously, there can be some variation out there.
Lastly, make sure that all files associated with your bootanimation (ie. VZW_bootanimation.zip, Verizon_Thunderbolt_3sec.mp3, and default.xml) have permissions set to 644/rw-r--r--.
Hopefully you find this HOW TO helpful. If you have any questions, or if anything wasn't explained clear enough, let me know, and I'll revise it to make it easier to understand.