pronunciation after "s"

[Secondtongue]

Can any friend tell me if there is any rules for pronouncing the k sound when if follow a "s". I listened to several audio cds on pronunciation for words such as "discount", "discuss" etc. My impression is that for "discount" , the "k" is still pronounced as "k", but for "discuss", the "k" is pronounced as "g". There are numerous examples like that I was a little confused. Further, someone told me that after "s", the "k", "t", (voiceless consonant) would pronounced as voiced that it become "g" & "d", according to some rules.
Please enlighten me.

[susiedqq]

Hard "c" is pronounced like a "K" in:
dis-cuss
dis-count
dis-covery

But a "soft c" in
dis-cern
dis-cipline