“Voilà is used in English to call attention to something, or to present some kind of accomplishment. For example, if you worked all day to plant a beautiful garden, you might present it to the rest of your family with a sweeping arm gesture and an enthusiastic “voilà!” “Viola” clearly doesn’t make sense in this context, but at least senseless violas are better than senseless violins.
“Voilà” is also frequently misspelled in ways that don’t confuse it with another word (but do indicate a mispronunciation). I’ve seen it as wala, wa-lah, wah-lah, and similar variations. Still spelled wrong, but perhaps not as confusing. If you have trouble remembering how to spell “voilà”, remember that it Very Often Isn’t Lettered Accurately.