AFAIUI, flash space wouldn't be the limitation*. Remember that the whole root partition is on flash and that paritition is not 100% full. The problem would be that for the sound to play ASAP upon boot, the (presumably) .wav would need to be embedded within the kernel. as would the equivalent PlayWav routines. I don't think that the memory used could be kfree'd up again afterwards. (I could be wrong about that.)
If you're not too bothered about 'ASAP' and can live with a little delay then it might be possible to do it using a userland application on the root partition and the old preinit scheme to launch it before the player.

* The flash space that is limited is the small block that is allocated to store dynamic parameters upon powerfail/shutdown. A chunk of flash is split into many n blocks of this small size, the idea is that by using a different block each time we wont wear out the flash. (Remember that the flash has a limited write count, this effectively extends it n times.)
The root parition is only written to when we upgrade the player software or kernel, ie not (relatively speaking) often.