Actually it looks like it may have more to do with the naming of the folders within the boot animation. I'm downloading Virtuous ROM now to take a look at how their boot animation is setup.
If you have an XML Editor you can look in /system/customize/CID/default.xml. about 3/4 of the way through the file there is a section that lists which files are used for boot up and boot down.
For instance here is what it says in the default.xml from the stock rooted dna rom:
-<BootConfiguration><BootAnimation audiostart="vzw" audio="/system/customize/resource/Droid_DLX_Eye_Animation_Sound_Design_9.21.12.mp3" animation="/system/customize/resource/vzw_bootup.zip"/></BootConfiguration>-<ShutdownConfiguration><ShutdownAnimation fps="10" image="" image_png="/system/customize/resource/hTC_downanimation.zip"/></ShutdownConfiguration>
Notice the audiostart="vzw". If you look in the desc.txt file within the stock boot animation file it looks like this:
1080 1920 30
p 1 15 android
p 1 30 vzw
Here is a quick description of each row in the desc.txt file:
1080 1920 30 >> 1080x1920 Resolution, 30 Frames p/Sec as Default
p 1 15 android >> 1st part uses the images in the android folder and repeats them 1 time, then pauses for 15 seconds
p 1 30 vzw >> 2nd part uses the images in the vzw folder and repeats them 1 time, then pauses for 30 seconds. The folder vzw also triggers the boot animation sound as noted in the default.xml snippet above.
I hope this helps.
---------- Post added at 11:19 AM ---------- Previous post was at 11:05 AM ----------
In Viper ROM it lists the audio file, but there is no audiostart tag, so I wonder if it just starts playing audio when the boot animation begins
Here is the default.xml snippet from Viper ROM:
-<BootConfiguration><BootAnimation audio="/system/customize/resource/htc_boot.mp3" animation="/system/customize/resource/bootanimation.zip"/></BootConfiguration>-<ShutdownConfiguration><ShutdownAnimation fps="10" image="" image_png="/system/customize/resource/downanimation.zip"/></ShutdownConfiguration>