In the first Transformers movie we see Frenzy regenerate its entire body in a couple of seconds once it finds the Allspark. Whether Frenzy initiated the repair or it occurred simply by being in the presence of the cube isn't clear. But in either case why didn't the Allspark repair Bumblebee's vocal processor before Bumblebee shrunk the cube?
The all spark DID repair bumble bees voice projector (albeit as Transformers 2 stated only temporarily). The problem was only that bee was so used to his "own" method of talking that he just used that instead of trying to talk and thus only did talk at the very end of the 1st film when he had something important on his heart.
Why it was only temporarily repaired though was never stated as far as I know.