This all comes back to the traditional English pronunciation of Latin, where a long or stressed “i” came to be pronounced with the /aɪ/ diphthong, as in horizon and saliva, or miasma and hiatus. You can see this phenomenon at work in the town of Salida, Colorado, where the English-speaking locals now pronounce their town’s name /səˈlaɪdə/, instead of using the original Spanish-language pronunciation of /saˈliða/.
So depending on which part of speech it is, and the degree of assimilation, there are two distinct possibilities for the pronunciation of via in English:
/ˈvaɪə/ (let’s call this the “English” pronunciation)/ˈviːə/ (let’s call this the “Italian” pronunciation)
However, that does not mean they are necessarily interchangeable, as there is some distinction here between the noun and the preposition. The preposition is more likely than the noun to have an English diphthong in it; that is, to use the first pronunciation, the “English” one. The noun is much more often these days to be of the second, “Italian” pronunciation. This may be because most “vias” one encounters are from Latin, Italian, or Spanish, where the word is still used.
For example, in Louisville, Colorado, there is a road named “Via Appia Way” [Google map]. (Yes, its first and last words mean the same thing.) That road always gets the second pronunciation here, never the first. However, when speaking of a bus or taxi route that went “via Via Appia”, one might well use the first pronunciation on the first of those two words and the second on the second, making its route run /ˈvaɪə ˈviːə ˈæpiːə/. No one here ever says /ˈvaɪə ˈvaɪə ˈeɪpiːə/ making it sound like English across the board.
Similarly, “foreign” (unassimilated) terms like the Via Dolorosa and the Via Lactea (that is, the original Milky Way), or places that have a road named Gran Vía, always have the “Italian” pronunciation, not the “English” one. That is why you have Coloradans saying their Via Appia that way.
On the other hand, a viaduct is always and without exception a /ˈvaɪədʌkt/ in English. Other, less common words that work this “English” way include:
- Something that is viable is always
/ˈvaɪəb(ə)l/, and the derived viability that accompanies it is always /vaɪəˈbɪlɪtɪ/.
- The trade-name Viagra is
/vaɪˈægrə/, rhyming with Niagara Falls’ /naɪˈægrə/, but more “viably”.
- A vial is always a
/ˈvaɪəl/, even(!) when somewhat archaically spelled phial (which, after Stephen, is the only other word in English whose ph is invariably /v/).
- A viator, meaning a wayfarer, has the somewhat unusual double-diphthonged pronunciation
/vaɪˈeɪtər/.
- The Ecclesiastical term viatica
/vaɪˈætɪkə/ is the plural of singular viaticum /vaɪˈætɪkəm/. A viaticum is the Eucharist given during Last Rites, or more generally, moneys or provisions given for travelling.
- Viands, a fancy name for provisions or victuals (“viddles”), are
/ˈvaɪəndz/.
- The rare viameter is
/vaɪˈæmɪtər/ (for its meaning, think odometer or pedometer), which has a feminine rhyme with diameter /daɪˈæmɪtər/.
- The pronunciation of viaggiatory is unrecorded, and it is not a common word. Perhaps
/vaɪˈædʒəˌtorɪ/, or if you are feeling especially naughty, even /ˈvædʒəˌtorɪ/.
That’s because when stressed and assimilated, spellings like via‑ and vio‑ have a /ˈvaɪə/ sound, like in violin or violence. Only in unassimilated terms like violino piccolo do you normally get the “Italian” pronunciation. Normally, a word needs to have a ‑veo‑ spelling in English for the /viːə/ pronunciation to prevail, as in alveolar or foveola.
