Sounds with breath after S

As I checked it on Antimoon, there says the sounds with breath (k/p....) are pronounced with less air breathing out from the mouth.

I knew it well, some body told me that the sound after s can be replace by ..Example: Scold -> (dictionary) ->skəuld
Should it be read as sgəuld? What's the difference?

I could give a short answer, which is that aspiration does not operate in certain consonant clusters, particularly those starting with /s/, in English, but that is really scratching the surface of the matter.

The matter is really that voicing of obstruents in English is not phonemically intrinsic or even a primary means of indicating distinctions between pairs of phonemes that shall be called "fortis" and "lenis" (aside from the phonetic ideas of fortis versus lenis, which are related to but separate from such), commonly and inaccurately called "voiceless" and "voiced", despite everything one may have been taught about English. English is not like, say, French or Spanish in this regard, and voicing seems to be less phonemically fixed in English than in Standard German or Dutch.

In this regard it is probably best to treat English more like Upper German dialects, Icelandic, or Faroese, where it simply lacks any inherent phonemic voicing of obstruents. Similarly, it is probably best to treat voicing of obstruents in English as being *allophonic* in nature, where lenis obstruent phonemes assimilate to adjacent sonorants and *become* voiced as a result (but at the same time may rather assimilate to adjacent fortis obstruents, and thus not become voiced).

The fortis-lenis distinction in English is actually highly multifactorial in nature, and there are phonetic cues that are more strongly distinctive in most positions than voicing for indicating the fortis-lenis distinction. However, though, just what phonetic means said distinction is communicated with, aside from voicing, varies depends on the position in a word of the given phoneme, as given below:

At the start of the onset of a word-initial or stressed syllable: aspiration*, consonant length, preceding vowel length**
Other onset positions: consonant length
Intervocalic positions: preceding vowel length, consonant length
Postvocalic positions: preceding vowel length, preglottalization*, consonant length

* only applies to plosives, and not to fricatives.
** if preceded by another word

As you can see here, as in clusters like /sk/ and /sp/ at the start of a word, the /k/ or /p/ falls in a position where the only thing distinguishing them from /g/ and /b/, respectively, and thus /sg/ and /sb/, is consonant length alone. As the /g/ or /b/ would fall adjacent to a fortis obstruent, their voicing assimilation to the following vowel would be prevented, thus preventing it from being distinctive. Consequently, /sk/ and /sg/, and /sp/ and /sb/, are all but neutralized, except for a small consonant length difference which most people are likely to miss. (Note that I would analyze such cases as being /sk/ and /sp/ in normal English words, as the length of the plosive is consistent with it being from a fortis phoneme.)