In my first post about accents of Spanish, I said I’d go into more detail about aspirated S. This post breaks it down and provides examples.
To recap, “S aspiration” has to do with the pronunciation of /s/ when it is syllable final, especially when followed by another consonant sound, and, in most S-aspirating accents, also when followed by another vowel sound.
To start getting into specifics, /s/ is pronounced h when followed by most plosives (namely /p b t d g/), the affricate /tʃ/, a nasal (/m n ɲ/), /l/ and /r/; and in accents that do so, also before a vowel. Examples:
esperar / los pibes ehpeˈɾaɾ / loh ˈpiβ̞es
esbirro / las buenas ehˈbiro / lah ˈβ̞u̯enas
esto / los tipos ˈehto / loh ˈtipos
desdecirse / los dados dehð̞eˈsiɾse / loh ˈð̞að̞os
esgrima / los guantes ehˈɣ̞ɾima / loh ˈɣ̞u̯antes
tres chicas tɾeh ˈtʃikas
mismo / los mismos ˈmihmo / loh ˈmihmos
desnudo / los nenes dehˈnuð̞o / loh ˈnenes
los ñudos loh ˈnjuð̞os
isla / dos limones ˈihla / doh liˈmones
es raro eh ˈraɾo
los otros loh ˈotɾos
To make things more interesting, this h often becomes a voiced ɦ when both preceded and followed by voiced sounds. So, this happens before a vowel and before /b d g m n ɲ l/. In my experience, for some reason, it is less likely to happen before /r/.
Before /k/, it assimilates to its place of articulation and is pronounced as a velar x. Examples:
escudo / los cuises exˈkuð̞o / lox ˈku̯ises
Finally, before another fricative, it assimilates fully to it. The result is that the following consonant is geminated; though the gemination can also be lost.
fósforo / los fuegos ˈfof(ː)oɾo / lo ˈf(ː)u̯eɣ̞os
descenso / los santos deˈs(ː)enso / lo ˈs(ː)antos
las llantas la ˈʒ(ː)antas
los juegos lo ˈx(ː)u̯eɣ̞os
I should note that gemination, with its possible simplification, of two successive s sounds, is also present in accents without S-aspiration. Likewise, in accents that feature distinción (differentiation of /s/, spelled S, and /θ/, spelled C/Z), two successive θ sounds suffer the same fate. We could lump these together with the rule that also applies to successive a sounds in the phrase la armada Argentina, and classify them as analog processes of simplification of successive identical sounds and of geminate simplification, applying both in linking and within the same word.
And that’s about the way it goes. Some caveats: for these examples, I am referencing my own accent (from Buenos Aires), with added S-aspiration before vowels – in other words, the accent you see in the examples is found somewhere between mine and my parents’ (they’re from Rosario). This accent also has a single phoneme /s/ for both the spellings S and C/Z, meaning that the syllable-final sound spelled with Z is the same as the one spelled with S, and is also accordingly aspirated in the contexts outlined above. Some examples would be diez cosas, pez espada, cruz de Malta.
S-aspiration does not usually happen with medial intervocalic /s/, because, following Spanish syllabification rules, that /s/ is syllable-initial. A possible exception to this can be, for some speakers, the word nosotros; and intervocalic syllable-initial S-aspirating is also possible in, for example, Paraguayan Spanish.
On a final note, I will mention that S-aspiration is very widespread. It is found in the south of Spain (roughly), the Canary Islands, and vast swaths of the Spanish-speaking Americas, including (but not limited to) most of Argentina, Uruguay, Chile, Paraguay, Central America, the Caribbean, and parts of Peru, Ecuador, Colombia, Venezuela, and Mexico.