I learned in English class that you should change pronunciation, k to g, t to d and p to b, that is immediately after s in spoken English, like express -> exbress and skill -> sgill. I am using several dictionaries, in their real person pronunciations, I heard exbress and discount, not disgount. When do you change pronunciation and when not, is there a general rule? Similar words include roster, exchange, steam.
It's true that the 'c' in 'disco' has the less harsh 'c' sound you are describing. I suppose it could sound like a 'g' to a non-native speaker.
However, 'discount' is the word 'count' with a prefix, and different emphasis may be put on each part of the word depending on whether it is being used as a verb (eg "do not discount the possibility') or as a noun (eg "I visited the discount store"). When used as a verb, you will find that more stress is put on the prefix, resulting in the 'c' sound also being pronounced more distinctly.
Similar words include disconcerting, discontinued, misconstrued etc.