Thanks a lot for all the effort you spend on this course! Now I do have voice sound and I can study on. I made a typo: ‘vikend’ instead of ‘víkend’ and the program even recognised that! Now I can study Czech with more confidence. I know I will be corrected on ALL my mistakes. And I can study pronunciation. Thanks again
Well.... You can.. The only reason, why "dobrý víkend" is somehow unthinkable, is .. habit. So you can try to change the habit and maybe people will follow you and in next years and perhaps centuries your invention will prevail. "Dobrý víkend" is completely correct, but no one would ever use it. And it is true that "Hezký víkend" is maybe little bit less standardized and less rigid greeting than for example "dobrý den", so it is not so essential to learn. For example you could say "Užijte si víkend" (Enjoy your weekend) - this is even more frequent (IMHO).
Certain consonants change their pronunciation when they are followed by certain other consonants. This is discussed in detail in one of the early Tips & Notes write-ups. I believe it is the Hello Skill; click on the Skill circle, then click on Tips link to view the T&N.
Czechs (and other Slavic speakers) find it pretty much impossible to pronounce the combination /-zk-/ and most other combinations where one consonant is voiced and the other voiceless. Voice assimilation dictates that all consonants in a cluster are either voiced or voiceless depending on the last consonant (with some exceptions). So "-zk-" is pronounced as /-sk-/, "vztah" is pronounced /fstach/, "zpěv" is pronounced /spjef/, but "sběr" is pronounced /zbjer/, etc. More in the Tips&Notes mentioned by BoneheadBass.