Jack MacGowran (Burke Dennings) bit the dust on January 30, 1973, from the flu. Other “reports” said he drank himself to death or he had a heart attack. Nope, just the flu from what I can find on him. The date he died also let’s us know how long The Exorcist was in production because the film was released on December 26, 1973.
You know it’s funny that you mention this point about Burke’s voice. Jack MacGowran had a fairly small part in David Lean’s Doctor Zhivago. If anyone here has seen the film he is the guy who politically disagrees with Zhivago during the sequences taking place in WWI and he says about Zhivago, “God rot good men.” His voice in that film sounds very ordinary in comparison to Burke’s voice which does sound similar to Mercedes in raspy timbre. Maybe because MacGowran was sick, but Friedkin and co. were really lucky that they had completed everything with MacGowran while they were still working on the film.
There’s are small snippets from The FoG of Mercedes McCambridge on this site. Unfortunately, and I have no idea why, McCambridge was cut from The FoG altogether.
Here’s the unedited version (McCambridge appears at the end) here.