This is not news. The louder one sings the more the vowels flatten out. Also the higher.
Verdi and the other bel canto composers always put very light orchestrations under crucial passages they wanted to be sure would be understood.
Did you know the Life of Verdi is now available on DVD? 4 discs and about 7 hours running time, it aired originally on PBS years ago, then on Bravo and 4 hours were added to the DVD. You can get it for a great price at deepdiscountdvd.com, though our new sealed copy did have some annoying skips and pauses on some of the discs it was well worth watching.
I've just ordered the newer DVD of Don Carlos sung in French as he evidently meant it to be sung, can't wait to see it.