All information comes from the outstanding editors over at AIO wiki, I'm just transfering it here :)
see also: https://www.aiowiki.com/wiki/Adventures_in_Odyssey#The_.22Split.22_era for more info
The Voice of Whit[]
In 1994, the voice of Whit, Hal Smith, died unexpectedly of a heart attack. This unfortunate event left the AIO team with a predicament as the voice of AIO's main character was gone.
A number of yet-unaired episodes had been recorded, including #266: “It Began with a Rabbit's Foot”, which was Hal Smith's final episode he recorded. This gave the AIO writers time to plot a course of action. Beginning in episode #274: “First-Hand Experience”, two of the show's main characters, Eugene and Bernard, left on a trip to California. This six-episode arc, which followed Eugene and Bernard's trip, gave the writers over a month to decide the fate of John Whittaker back in Odyssey.
Eugene and Bernard returned to Odyssey in episode #280: “Gone...”, and learned that Whit has taken a position in Israel with the Universal Press Foundation's Missions Board, and that due to the urgent nature of the vacancy, Whit was scheduled to leave that day. When Eugene and Bernard discovered Whit left; they raced to the airport, only to have Whit depart just after they arrive. The AIO staff used several clips of Whit from past episodes, in addition to portions of a tape Hal Smith sent to the AIO staff shortly after the death of his wife, to create the scenes with Whit.
Whit returned in 1996 voiced by a new actor, Paul Herlinger. His first episode was, appropriately, #355: “The Search for Whit, Part 1”. By that time in the series, his archaeological trip was over and he found another Odyssey character to take his place. Whit has never left Odyssey since.
In late 2008, Paul Herlinger informed the AIO Team that they should begin searching a replacement for Whit, because he was having health problems and did not want the team to be stranded without a voice for Whit once again. The AIO Team held auditions and ultimately decided Andre Stojka to be a good replacement. Herlinger's last episode was #TTC08: “The Final Call”, and Stojka's first was
Some fans found it hard to adjust to a new voice actor, and asked for Herlinger to return. nfortunately, Herlinger passed away in February of 2010, reaffirming that Stojka was to be the voice of Whit.