1) To disable the start up sound, in the /etc directory, rename the file PowerOn.wav to PowerOn.wav.bak.
2) To disable the shutdown sound, in the /system/media/audio/ui directory, rename shutdown.ogg to shutdown.ogg.bak.
I did this using a rooted phone with ADB. I am sure there may be an easier way if someone would like to provide further instructions regarding this.
I hope this info helps someone, those sounds very super-loud and annoying! If this info is available elsewhere, let me know and I will remove this post.