I used to have start up sounds a while ago before I made the TTS Clock script. If you just want a single start up sound you can add a hijack config.ini entry like this (assuming you have TTS Clock installed which installed pcmplay for you):
;@EXEC_ONCE /drive0/var/pcmplay < /drive0/var/startupsound.pcm

For random sounds, you can create a bash script like this:

#!/bin/sh
number=$RANDOM
let "number %= 7"
case "$number" in
'0')
/drive0/var/pcmplay < /drive0/startup/shortcircuit.pcm
;;
'1')
/drive0/var/pcmplay < /drive0/startup/welcome.pcm
;;
'2')
/drive0/var/pcmplay < /drive0/startup/playgame.pcm
;;
'3')
/drive0/var/pcmplay < /drive0/startup/homeranykey.pcm
;;
'4')
/drive0/var/pcmplay < /drive0/startup/homer7dohs.pcm
;;
'5')
/drive0/var/pcmplay < /drive0/startup/hal9000.pcm
;;
'6')
/drive0/var/pcmplay < /drive0/startup/butthead.pcm
;;
esac

Save the bash script as a unix style text file, give it permissions, and also call it from a config.ini entry.

Also if you do this, it would be a pretty good idea to run setvol from the config.ini before any of the custom sounds, as you probably don't want the volume to be 0 or 100%:
;@EXEC_ONCE /drive0/var/setvol