> I find it fascinating that languages have different sets of sounds. For example, in English p and b are two different letters, and pat and bat are two different words. But other languages don't distinguish them, and their speakers really wouldn't notice a difference if you said pat or bat.
This is related to a question I've been wondering about for a long time.
One of the first stages in language processing is converting a sound's raw physical waveform into a sequence of phoneme tokens. Different languages divide different sounds differently -- as you note, [p] and [b] are distinct in many languages, but not all of them.
What I'd like to know is how nonexistent sounds get classified. Take [θ], the thorn sound. It is rare, but physically similar to several other common sounds -- the stop at the same place of articulation would be [t̪], the closest nearby tongue-based fricative is [s], and [f], while not tongue-based, is even more similar.
All of those are attested substitutions for the difficult [θ] sound. And they appear to be predictable -- English-speaking babies reliably substitute /f/, which they can pronounce, for /θ/, which they can't. Mandarin speakers hear /θ/ as /s/. Cantonese speakers hear it as /f/. (As, I believe, do Russian speakers.) In Ireland, the sound is replaced by some kind of /t/. But all of the languages I just mentioned offer the full set of /f/, /s/, and /t/ as available options.
To a certain degree this is conventionalized. I'd believe that modern Irish pronunciation is based more on what you hear from other Irish people than on a difficulty with thorn. And in English, the Mandarin sound /tɕʰ/ has sometimes been recorded as "ts" and sometimes as "ch", and if you pronounce it to untrained listeners and ask them what they heard, you'll get a mix of both answers. (Today it is less helpfully spelled "q".)
But nobody trains babies that if they can't produce a correct /θ/, the proper thing to do is to fall back on /f/, because /s/ would be a faux pas. The babies just do their best, and they all independently agree that their best approximation to /θ/ is /f/.
I think it should be possible, if you know the phonology of a language, to determine which phoneme (or possibly phonemes) a particular foreign sound would be understood as by a speaker of that language. As things stand today, it isn't -- we know that different languages will cause the same sound to be heard differently, and we know experimentally what the likely options are for a given sound, but we can't predict an individual mapping. And there's enough consistency in what people hear that we _should_ be able to do that.