It is rare for an native (first language) American English speaker to have any idea how the orthography for Brazilian names to work, so they will pronounce them with the rules of American orthography:
- João Gilberto: dzhoh ow dzhill bear tow
- Astrud Gilberto: ass truhd dzhill bear tow
- Bebel Gilberto: bee buhl dzhill bear tow
- Vinicius de Moraes: vin ee shee us duh muh ray ess
- Antonio Carlos Jobim: an tow nyo car lohss dzhow beem
- Vivo Sonhando: vee voh sun han doh
(yes, the transcription is in 'sound it out' English orthography; IPA would take forever)
It is obviously not expected that an AmE speaker would be able to pronounce the nasal, but even though they could pronounce the 'ny' in Brazilian 'nh' or the initial 'g' has glottal aspirate 'h', Portuguuese (Brazilian at that) orthography is pretty rare, and so could not be expected to be known at all. (Spanish is popular enough for AmE speakers, and even there the orthography is not so well known).
As to news- or sportscasters, often they are given a close enough approximation so that the names aren't agonizingly butchered.
So which pronunciation is standard for the [ʊ] sound? Rounded or unrounded?
Certainly there is some rounding, but because roundedness is not phonemic in this position, there is also considerable variation in how much of it actually occurs in any given word and speaker.
For example, you will find that it is generally somewhat more rounded in pull and full than it is in put and foot respectively. That’s because having an r or an l right next to it rounds it off a bit — which is why it is a bit more rounded in root and rook than it is foot or cook. Same with rookie versus cookie, where the first version is a bit more rounded than the second. And of course, a w helps: compare how wool is even more rounded than full, and also moreso that wood.
I believe English has no words with [ʊw], as that seems redundant. However, it can occur in phrases, especially in some dialects, where something like I knew it full-well may approach that.
However, it is still perceived as the very same phoneme in all those words and cases I’ve just listed above.
Correction — or not
I said that I thought English had no words with [ʊw] in them. And at the end of the day, I still believe that. However, I have discovered that grepping the OED yields the apparent existence-proof counterexample of Rauwiloid, which means:
A proprietary name for a hypotensive preparation containing a number of alkaloids extracted from Rauvolfia serpentina.
You also have compound words whose first element ends in [aʊ] (rather than [aw], as it is sometimes spelled) connecting to something that begins with [w], and which have in effect a “double w” in them, you expand the list to include such things as:
bow-wow, powwow, skeow-ways, wow-wow
Finally, if you consider the sound in words like no and micro to be
an [oʊ] diphthong rather than [ow], then you get all these, most of which were originally compounds of some sort:
froward, frowardly, frowardness, glow-worm, Holloway,
hollowwort, Howeitat, Khowar, meadow-wink, microwave, microweld,
Moldo-Wallachian, nowise, Oldowan, Parowax, powan,
shalloway, slow-worm, swallowwort, werowance,
yellow-wood, yeowoman.
For example, yeowoman theoretically yields /ˈjoʊwʊmən/, at least in North America. Still, there is a reasonably convincing argument to be made that that one is better written as simply /ˈjowʊmən/.
Slightly less uncommon is nowise, which is a compound of one word ending in a diphthong connected to another starting with a triphthong, so /ˈnoʊˌwaɪz/.
But I am still highly dubious of the existence of [ʊw], because I think it fuses into the semi-consonantal glide, [w]. After all, nowise and no eyes are homophonic, so I think this idea of [ʊw] is very hard to justify, and so I stand by my initial statement.
Even towel is usually pronounced with just one syllable, /taʊl/, thereby rhyming with cowl /kaʊl/. Even with folks who work very hard to put two syllables into that, with /ˈtaʊ.wəl/, I submit that you could write that /ˈtawːəl/ and avoid the whole controversy of whether a semi-vowel/semi-consonant/off-glide is really /ʊ/ or really /w/. However you write it, it seems like the same sound to me, such that bisyllabic towel just has a geminate [w]: /ˈtaw.wəl/.
Best Answer
The pronunciation of Greek letters by scientists isn't very different from the pronunciation of the Greek letters in the respective countries: American scientists pronounce them pretty much the same way the general American population does, and so on.
So your question is actually about why the English pronunciation of Greek letters, and the answer is that it is based on (but not always actually very close to) the reconstruction of the Classical Greek pronunciation by Erasmus in 1528 and by John Cheke and Thomas Smith around 1540, which were adopted in schools. This pronunciation underwent some change along with the rest of English during the Great Vowel Shift, and a re-reconstruction in the mid-19th century brought it back in line (incompletely) with Ancient Greek. The Wikipedia page on Pronunciation of Ancient Greek in teaching has more details.
For sake of completeness, here's a (very incomplete) table showing the pronunciation in American English, British English, Ancient Greek, and Modern Greek. I've rearranged the alphabet to put sort-of rhyming letters together, but all letters are there.
[Disclaimer: Many of the entries may be terribly wrong. The American and British IPA entries are based on the article English pronunciation of Greek letters, the "pseudo-phonetic spellings" are from here and here. The Classical and Modern Greek pronunciation columns I made up, partly from Swedish Wikipedia, partly from piecing together each letter's pronunciation in this table, partly from here for Modern Greek, and partly on my own — and I don't actually know IPA.]
[Edit: This table has now been edited to correct the IPA and source Classical Greek pronunciations from the English Wikipedia.]
(FIE, FEE)
(FIE, FEE)
(KIGH, KEE)
(KIGH, KEE)
(SIGH, PSIGH, PSEE)
(SIGH, PSIGH, PSEE)
(ZIGH, KS EYE, KSEE)
(ZIGH, KS EYE, KSEE)
(EP sil on, ep SIGH lun)
(OOP sil on, YOOP sil on)
(AH mih cron, OH mih cron)
(OM ih cron, OH my cron)
(oh MAY guh, OH mee guh, OH meg uh)
(MYOO, MOO)
(NYOO, NOO)
(ROE, HROE)
(TOW rhyming with COW, TAW rhyming with LAW)
(TOW rhyming with COW, TAW rhyming with LAW)
Notes:
I've made this community wiki so that someone can fix the errors or complete the table (including possibly myself if I regain the patience to finish this sometime!)