There are already third party programs which can play a sound during boot up. Do a search on the BBS to find them.

However, they cannot coincide with the boot animation. They can only happen after the boot animation. Here is why:

The boot animation and boot logo are loaded from flash memory so that they can give you something to look at while the player's disk drive spins up. The third party programs and the stored sounds only run from the hard disk, and therefore sound can only happen after the boot logos are done playing.

If you were able to write a version of the kernel that could play sounds, then it would be theoretically possible to load those sounds from flash and do what you are suggesting. But the flash memory is very scarce and does not have room for much of anything except the kernel, the boot logos, and the player settings. So even if you did get a version that could play the sound from flash, it could only be the shortest, simplest, lowest-quality sound imaginable.