This was from long ago, but I heard of it only recently:
IMDb :: Boards :: Untitled Transformers Sequel (2014) :: Gary Chalk should do a voice

There was a tiff between voice actor Gary Chalk and producer Don Murphy. The gist is that Murphy turned down Chalk for a voice role (claimed to be OP) in a live-action TF, which upset Chalk a lot. But that's all that was mentioned.

Can someone please clue me in on what really happened? Because it's a real shame that an established voice actor like Chalk doesn't return to TF, whether for the films or any other continuity. It does sound like a snub from Murphy, and I agree it wasn't deserved, but if he was volunteering to play Optimus Prime when the fans wanted Peter Cullen for the job then what could he expect? I'd just like him to bury any ill feelings that exist, if they do, and return to a TF feature (whether a film or a series).

 

Ah yes, the IMDB boards.

"They've avoided him, or forgot he existed for three films now. I would like Gary Chalk to voice one of the Transformers this time. He's a supporting actor, but usually best known for voicing Optimus Primal in BEAST WARS, and Optimus Prime in TRANSFORMERS: ARMADA. I know Peter Cullen is returning as Prime, but I hope Gary Chalk is given a voice role. Maybe as Ultra Magnus."

We don't know how things went down. We don't know when they went down. We don't know how Murphy spoke to Chalk, or if he did at all. Did Chalk even actually audition? Did the audition suck? Did they just not feel Chalk was right for the role? Was he too late? Was Chalk rude? Was Murphy rude? Is the scenario being blown out of proportion? We'll probably never know for sure what exactly went down. 99% of the time Hollywood is pretty tight-lipped about the true outcomes/intentions/specifics pertaining to events like this.

I'd also like to note:

Then there's apparently thousands of other voice actors who have been "forgotten" because they haven't been included in the movies. Not being cast in a movie means you're being avoided? Jeez. I haven't had any talent agencies come knockin' at my door in months! THEY'RE AVOIDING ME! How rude! Ugh!

On a slightly more upbeat note, whenever I watch RotF, I can't help but feel that the Captain Wilder of the John C. Stennis aircraft carrier role would've been perfect for a Garry Chalk cameo. He's got that hardened military look to him, and it's such a minor role that it'd be great for a walk-on-for-a-day shoot.