#29 - "Maybe your brain insists on replacing what you heard with a familiar sound ...".
Yes, this is exactly why most Vietnamese have such trouble learning English. They don't even HEAR all the sounds, especially the final 's' and other final consonants in a word. They are trying to listen to Vietnamese sound patterns and that's what their brains 'hear', instead of hearing English.
Indeed many languages have similar sounds and so the brain and your body's muscle-memory often try to default to that.
Or if the language you are learning has two sounds and your own language has one that is somewhere in between, you make the in-between phoneme and speakers of the language you are learning mis-hear you.
e.g., the English /b/ and /v/ sounds (/b/ is a bilabial plosive, using both lips, with the sound exploding out; /v/ is a labio-dental frictive, using a lip on the teeth with the air coming out with noise produced by friction; both are voiced), while Spanish has a bilabial frictive a sound in between made with both lips but the air coming out with friction (e.g. in Los Viejos [spelling uncertain])
So a Spanish speaker sayng /b/ sounds like a /v/, and saying /v/ sounds like a /b/ - in fact for both, they are producing a sound in between.
I have struggled with consonants in several languages. My native language is English which is rick on vowels but impoverished in consonants.
I have tried to pick up bits of Caucasian languages: after much practice and help I can now make a reasonable fist of the ejective / aspirated distinction in Georgian but I have not yet got the hang of the 9 different k sounds in Circassian (Abkhaz dialect). It took me a long while to detect the glottalised / not glottalised consonants in Arabic, or the similar distinctions in Punjabli. I found the Punjabi retroflex consonants quite easy to hear and form despite the lack of corresponding sounds in English.
I couldn't master the diffiuclt sounds by listening alone. I need a combination of a native speaker of the foreign lanaguage and someone who can explain exactly what parts of my mouth I have to move where. I then try to follow the mouth instructions, until the native speaker says "right, you got it". For a while, I can form the distinct sounds correctly but am still unable to hear the difference when spoken by someone else. Gradually your ear gets attuned.