Well I'm surprised that Takei was asked too do the audio book version, since Sulu's hardly been in the novel. Sulu basically made a quick cameo and that's it. It would've been better had the audio book featured Shatner with Nimoy or even Kellerman or the actors that playd Kelso and Mitchell.
The idea of the early ST audios was to produce them as financially viable as possible. Nimoy's participation was only secured by promising him that his part would be minimal, presented as "Science Officer Logs", scattered through the main narrative. Shatner was way too expensive and too busy. The "T.J. Hooker" years.
The first five audios were:
"Star Trek IV: The Voyage Home" by Vonda N. McIntyre, read by Leonard Nimoy and George Takei, 1986, 90 min.
"Strangers from the Sky" by Margaret Wander Bonanno, read by Leonard Nimoy and George Takei, 1987, 90 min.
"Enterprise: The First Adventure" by Vonda N. McIntyre, read by Leonard Nimoy and George Takei, 1988, 90 min.
"Web of the Romulans" by M.S. Murdock, read by Leonard Nimoy and George Takei, 1988, 90 min.
"The Entropy Effect" by Vonda N. McIntyre, read by Leonard Nimoy and George Takei, 1988, 90 min.
"The Entropy Effect" was the most unusual, in that the audio uses the Kirk/Spock cover art from the novel "Triangle". Sulu's subplot (and his new mustache) had been abridged right out of the audio, so it was unwise to use the original artwork. So Takei narrates, but his character isn't even mentioned.
George Takei had been the actor approached to do that first one, and possibly his contract gave him first refusal on new ones. Nimoy got top billing, but participated less. I recall fans asking (in "Starlog"?) why James Doohan, more renowned for voiceover work than Takei (eg, Doohan's many accents and characters in Filmation's TAS) and, sure enough, Doohan and Nimoy did the next three:
"Yesterday's Son" by A.C. Crispin, read by Leonard Nimoy and James Doohan, 1988, 90 min.
"Final Frontier" by Diane Carey, read by Leonard Nimoy and James Doohan, 1989, 90 min.
"Time for Yesterday" by A.C. Crispin, read by Leonard Nimoy and James Doohan, 1989, 90 min.
Takei and Doohan then alternated on the next few. Then Doohan went solo (with "Prime Directive") when Nimoy stopped doing them and the audios doubled their length to 180 mins. Participation from William Shatner finally occurred with a "25th Anniversary Audio Collection" (1991) of best-selling rereleases; it was a brief new introduction.
The first audio to focus on a
guest ST artist was "Faces of Fire" by Michael Jan Friedman, read by Bibi Besch in 1992. Walter Koenig was asked to do a Chekov-heavy novel ("Windows on a Lost World") in 1993. And so on...