About Obi-Wan's roar, when I was young I always thought that it was the scream of the Sandpeople who got scared off by Obi-Wan's presence. ;)
I know there has been silly EU explanations of this beeing Ben imitating a Crayt Dragon or something but in the original sound-mix they actually used the same sound effect they used for the Dewback earlier in the film, so maybe that's why they opted to change it. Nevertheless, the new sound effect sounds terrible as the rest of the audio mix.
I found this info in an old SW magazine:
"In Lucas' early draft, the Jedi Knights let out an ear-piercing scream just before they attacked. This idea survived in the call that heralds the arrival of Obi-Wan."
Can someone confirm if this info is true? I haven't read the earlier drafts in a long time, and I cannot remember that part.