Well sorry my method doesn't work for the startup sound. It worked from removing the SingTel AVI though. Go to your terminal and type this in:

% devel-su
[password]

# cd /etc/mwelcome.d/
# ls

If there are any numbered files here, it means your carrier has placed them in here. You can use the following command to get rid of the video:

# mv filename.conf /home/user/MyDocs

We are moving so in case something doesn't work, we can move it back. Reboot your mobile and your avi should be gone. This won't work for the base Nokia vid though. The filename for the Nokia video is default-nokia-hands.avi.