The thing is, S, P and K are all unvoiced. I really see no reason why P or K would change in those words to voiced consonants - that would be even more difficult to pronounce. Wednesday has a Z sound, thanks to the N sound (or more correctly, the -dnes- sound all fused together). Instead of trying to decode the words technically (I think you're just causing yourself more confusion), you can find audio examples via: http://howjsay.com/ http://www.forvo.com/ http://www.ivona.com/

I think your understanding of the pronunciations for Spain, Basket and Wednesday are correct. Wednesday is an odd word. It is pronounced in a manner (that I think you clearly recognize) that is different from its spelling. Wenz----day is what you will hear in the USA. Basket has an "a" sound like the "a" in Fat, Cat, Sat. Spain has the long "ai" which sounds like the Long "A" sound in Pay, Say, Day. We refer to this as "A" saying its name. .