You should pronounce the "c" is both cases. the expression "dis donc" is a familiar one and even if we (french) don't pronounce the "c", it is a mistake :)

You can try listening to native French speakers here: http://forvo.com/search/donc/

Don't listen to Anne-Laure, both pronunciations are right, but you have to stick to everyday usage. I'd say that when you say "donc" at the beginning of the sentence you always pronounce the "c". In all the othey cases you can choose whether you pronounce it or not, like "dis donc", "quoi donc", and so on.

dis donc的时候，最后的k发音很轻，有时候被省略了.....这种现象在各种语言里都有。比如在汉语中，“你不知道”的“你”经常被省略成“n”，而不是“ni”