Ok, this is what you do.
1. Find an mp3 file you like (MP3 file, not an Ogg Vorbis file)
2. Name it "boot_audio.mp3" (case-sensitive)
3. Copy it into /system/media/ on your actual phone (NOT in the ROM zip unless you want to have to reflash your zip)
4. Change the permissions of the file to rw-r--r--.
5. Go to Settings > ROM Control > General UI and make sure that "Disable Boot Audio" is UNCHECKED