Well there are a few rules on the "whispered sounds" but they aren't always true.
The most common whispered sound is "SU" as is used in DESU and MUSUKO. It's very easy for SU to sound more like S then SU. But it isn't always the case as you pointed out in MUSUME. I think it's just easier to do it in MUSUKO than it is in MUSUME.
There is no real science behind it but you can get more information on an oldie but goodie George and Keiko Show.
Their is a linguistic rule as far as japanese phonetics go, but unfortunately I forgot it. Something about the tendency for the 'u' sound to go unvoiced when its word final (at the end of a word) and before the consonant 'k' .... there's more but that's all I can remember for now. Good thing to know is, people don't speak languages perfectly alongside of the rules, it changes constantly, so its ok if you don't know the exact rule. Your best bet, I would think, would be to keep your ears open and just follow what you hear native speakers doing.
thanks. I remember watching the show earlier and your right George San, it did help I just thought that it was exclusive to certain characters not all. I guess I will have to keep my ears open just like what kemushichan said :)