In the production notes of The Original Hitchhiker's Radio Scripts, Douglas Adams explains that the above jingle should be rendered as follows:
Once a tune has been worked out, the accompaniment should be played on a very echoey synthesizer whilst the two million robots sing exactly a flattened fifth out of tune. It will sound more ghastly than you can possibly imagine.
The BBC production staff had a terrible time trying to get exactly what Adams wanted for the following reasons:
1. "It is impossible to have two million people singing anything and still make out what they are singing."
2. "It is even more impossible if these two million are robots who are going to be hard to understand anyway."
3. "And when it comes to having them singing a tone flat as well then impossible is no longer a strong enough word and we have to resort to being about as likely as being able to extract sunbeams from cucumbers."
Eventually, they resorted to pulling half a dozen people out of the hallways of the Radiophonic Workshop to stand in for the two million robots.
Frighteningly enough, the narrator tells us that the actual jingle sounds "something like this," but "slightly worse."