It's been a while since I have read Stranger in a Strange Land, but was the
cause or nature of the deaths of the Envoy's crew revealed?
Near the end of chapter 4 Ben tells Jill one part of the story that he got from someone who read the Envoy log.
Dr. Ward Smith delivered his wife of child by Caesarian section - and she died on the table. He seems to have worn his horns complacently until then. But what he did next shows that he knew the score; with the same scalpel he cut Captain Brant's throat - then he cut his own.
Other than that, I don't think how they died was specifically mentioned. It is clear from one report by the Champion:
Rocket Ship Envoy located. No survivors.
That all of the Envoy's original crew was dead.
There were 8 people on the Envoy, the original mission (part one, ch I):
Captain Michael Brant, commanding—pilot, astrogator, relief cook, relief photographer, rocketry engineer;
Dr. Winifred Coburn Brant, forty-one, semantician, practical nurse, stores officer, historian;
Mr. Francis X. Seeney, twenty-eight, executive officer, second pilot, astrogator, astrophysicist, photographer;
Dr. Olga Kvalic Seeney, twenty-nine, cook, biochemist, hydroponicist;
Dr. Ward Smith, forty-five, physician and surgeon, biologist;
Dr. Mary Jane Lyle Smith, twenty-six, atomics engineer, electronics and power technician;
Mr. Sergei Rimsky, thirty-five, electronics engineer, chemical engineer, practical machinist and instrumentation man, cryologist;
Mrs. Eleanora Alvarez Rimsky, thirty-two, geologist and selenologist, hydroponicist.
Three of them are accounted for (part I, ch IV):
“It’s a nasty story. I got that much before my informant sobered up. Dr. Ward Smith delivered his wife by Caesarean section—and she died on the table. What he did next shows that he knew the score; with the same scalpel he cut Captain Brant’s throat—then his own. Sorry, hon.”
Jill shivered. “I’m a nurse. I’m immune to such things.”
“You’re a liar and I love you for it. I was on police beat three years, Jill; I never got hardened to it.”
“What happened to the others?”
“If we don’t break the bureaucrats loose from that log, we’ll never know—and I am a starry-eyed newsboy who thinks we should. Secrecy begets tyranny.”
As far as I can tell, there's no further mention of the other five crewmembers. Not a very satisfactory answer, but there it is. Unless Heinlien answered in in some other venue, the fate of the rest of the Envoy's crew remains unknown.