They seem to be wav files under /usr/share/sounds . Backup the original file(s) first so you can restore them if you decide to experiment. I don't know if they have to be a certain size / length etc. but replacing the email sound file (which was called ui-new_email.wav) doesn't sound too dangerous to me

I would do it by first editing a sound file you like on a computer, naming it the same as the email sound, and then open an SSH connection into the tablet (look up openssh on the forums if you haven't set that up yet) and copy the new file over the old one. But take a backup first and be careful with the SSH connection - that'll be root access which means that you can mess up things really well if you delete something important.

Hope this helps - have fun!