Freakin' phonemes

I once had a professor from Columbia who told us she couldn't hear the difference between the words "shit" and "sheet." She used this as a funny example while teaching us that humans are born with the ability to distinguish between all possible phonemes, or language sounds, but lose the ability to hear phonemes to which we are not exposed. The theory is pretty well supported but I'm not sure I bought her shit/sheet story, or a more recent teacher's bitch/beach story either. Now I am a believer.  There are two "ou" sounds in Brazilian Portuguese, au as in "ouch" and an ão sound that does not exist in English but is described as nasal. Unfortunately, au and ão sound the same to me and I can't replicate the difference between them. This means that when I try to say the word  pão, which means "bread" I actually say the word pao, which means "stick" or "wood"  but is a slang for "dick." It is uncomfortable for everybody and has started making me self-conscious in a country where one of our favorite foods is cheese bread.