I can't believe I never thought of this before - other people must surely have covered this somewhere?
The funny, beepy alien speech in The Boy From Space, well I'd always just assumed it was random, electronically generated noises strung together. But it's not - it's actual English speech, obviously scripted just like the normal dialogue was, being played backwards and mucked about with a bit!
It's not very easy to make it out on the TV film, because it's been mucked about with an awful lot, and there's all ambient sounds getting in the way. One of them that I tried playing backwards seemed to involve Peep-peep saying to Dan at the end "we fly through space together" or something along those lines, but it's generally quite indistinct.
However - on the LP recording of the story, the space speech is read out completely isolated, and only a bit peeped-up, so you can actually tell exactly what they're saying most of the time just by playing it backwards! Here's some examples:
Peep-peep's Father tries to tell Mr Bunting about the meteorite room. I can't quite make this one out, I think it's either "the meteorites are from space," or "the meteorites are not safe," or something like that. Anybody got any better suggestions? (Narrator's line is "And then at all the meteorites.")
So there you go - the actual space-men's voices!
If anybody can make out what's being said in any of the TV episodes, please post it here! I'll definitely be adding some of this to the site proper as soon as I find time to update it...