I emailed the customer support address on the site and asked what format, bitrate, etc. the downloads would be. Hopefully they'll answer within a few days.

(Off topic observation: It would be nice if the <sup> tag were allowed. Also <sub>.)

Mike K is not correct - the kilogram is not defined as the mass of a volume of water at 4°C. That definition would be a lot more unstable than the platinum-iridium object in the vault in France, because water likes to dissolve things. To nail it down you'd have to specify the precise composition of the water sample, the container, and the air above the water -- not to mention the pressure of the air. By the time you'd done all this work it would be easier to just use a lump of platinum-iridium. Which is what we do.

There are people working on redefining the kilogram to not rely on an artifact, the same way all the other basic SI units have been redefined, but the alternatives they are looking at tend to be things like "one kilogram is the mass of exactly 2.1507e+25 atoms of pure silicon 28". And we need to be able to write that number down to a lot more decimal places before it will do a better job than what we've got.

http://www.npl.co.uk/mass/faqs/kilogram.html is a good overview of the present state of play, the problems, and what people are trying to do about it.