Trivia: The movie is set in 1987. This is the same year the original "Transformers" cartoon series ended.
Trivia: The film was originally made and marketed as a prequel to the prior "Transformers" films directed by Michael Bay. After the film was released to a much warmer critical and audience reception than the prior films, it was confirmed that instead of a prequel, "Bumblebee" will instead serve as a sort-of "reboot" of the series, and that future films will serve as direct sequels to it, without necessarily acknowledging the continuity of the Bay films.
Trivia: Star Hailee Steinfeld wasn't born until 9 years after the film is set. She actually had to be given lessons on things like 80's lingo and how to properly use a Walkman.
Trivia: Both the shortest "Transformers" movie (coming in at under two hours) and the least expensive film in the series. (Costing under $130 million, as compared to the $150-$200 million the other films cost).
Trivia: Pay attention to the name tags and you will notice one of the junior agents is named Simmons, a major character in the films which take place after this one.
Answer: I couldn't find any other information, but honestly... Soundwave is barely in the movie. Probably just wasn't worth it for the production to pay Welker to come in just to record a few throwaway lines.
TedStixon
I considered that possibility as well, but since Peter Cullen reprised his role as the voice of Optimus Prime yet again for this film despite Prime's minimal involvement, it doesn't quite wash. Also, given Frank Welker's highly prolific voice acting career, it's unlikely he would have declined on the basis of pay or importance of his involvement.
zendaddy621
Soundwave was barely in the movie (he's literally only in a few shots) and only had one or two lines. As the other answer suggested, it probably just wasn't worth it for the studio to pay Welker to come in, or they just didn't feel it was necessary given that Soundwave was essentially just a blink-and-you'll-miss-it cameo. Optimus has a much larger presence, and Peter Cullen's voice is pretty synonymous with the character in the film series. Hence, it was worth it to have him return.
TedStixon