Voila comes from the root word Vois (to see) — and while many people do pronounce the “v” clearly, often the “v” is slurred and sounds like w. :)
Vwa-lah, Wa-lah, Wa-la: the Many Misspellings of voilà
Voilà is a French borrowing into English that has mostly retained is Francophonic pronunciation: \vwä-ˈlä\, or \vwah-LAH\. It is clear that the \v\ in the pronunciation of voilà is sometimes not heard, and this, combined with the mismatch between voilà’s spelling and pronunciation, has led to a number of misspellings of the word based on its pronunciation. We have seen evidence for vwa-lah, vwah-la, wa-lah, wa-la, wah-lah, wallah, and even viola (for those who remember the letters in the word, if not the particular order of them). While these misspellings are more common in informal writing, we have started seeing them in newspapers and other edited sources as well.