The problem is people do not have a bit of imagination. They've been spoiled. Always demanding answers for every small bit in the movie like its some kind of documentary for Chemist Students in a University. Its not. Its a freaking movie, and a suspense in that. Its not exactly star trek is it? Try putting all these details into 2 and half hours. That would have diverted the movie from the storyline and the concept of what Speilberg was trying to portray.
By the way heres the signalling sound:
http://www.jinn.myby.co.uk/wbpgs/tripodcall.mp3