The aspiration or loss of /s/ syllable final in spoken Spanish
The aspiration or loss of the /s/ sound in a final position of a word is a natural phonological feature found in many varieties of Iberian and American Spanish.
This language phenomenon is marked by an aspiration or dropping of the /s/ when speakers articulate structures like los niños, las casas or las tiendas grandes. An approximate phonological transcription for these structures may be: loh niñoh, lah casah and lah tiendah grandeh.
I’ve heard quite often people commenting about this issue in a negative way. Some of them think that some native Spanish speakers aspirate the /s/ due to pure linguistic laziness and that such speakers must make an effort to correct their speech. Whenever I can I point out to anyone making this type of comments, that this phonological phenomenon – like all other aspects of language – is neither wrong nor right. That speakers resorting to this kind of linguistic structures are just making use of language in a way suitable for them.
At the end of the day what really matters for speakers of a language quite independently from the conventional features such as standard pronunciation patterns, is that the linguistic structures being used in any given situation must be socially accepted in the setting where they’re using such structures.
Thus it doesn’t really matter whether some Spanish speakers say lah casah instead of las casas as long as they are mutually intelligible.
In the language classroom, however, students must be taught to pronounce the standard /s/ whenever necessary in their spoken Spanish.