I've tried it several times and I can't for the life of me work out how it's done. You don't need a sound card for this, just click the link and see if you can figure it out.
This will mean nothing until you've tried it, but the email reads (and I guess it's just a forwarded text because Nigel doesn't write like this, thank heavens):
"This stumped me. If you can figure out how she does it please let me know. I never even touched the cursor on my chosen number.. Once I did not even follow the directions, I just looked at the number and she still got it!"
Here's the link: click here to have a go.
About five minutes after I posted this, Mrs The Millbrooker (otherwise known as Mrs Very Clever Pants) told me she'd already seen it and worked out how it's done in a few minutes.