Factual error: Rob Base and DJ E-Z Rock's "It Takes Two" was not released until 1988. This movie takes place in 1987.
Factual error: "Never Gonna Give You Up" by Rick Astley is very briefly played; while that song was originally released in the UK in July 1987, it wasn't released in the US in any form, including to radio, until 1988.
Factual error: The term 'carjack' was not coined until 1991.
Factual error: When they pull up to the cliff "It Takes Two" is playing on the radio, this song was released in 1988 from the album which was released the same year. and the film is set in 1987.
Factual error: In the shots of the Golden Gate Bridge with downtown San Francisco in the background, the Salesforce Tower is visible, but this building didn't even break ground until 2013.
Factual error: During the first scene with the family, dad is wearing an Izod shirt. At that time it was still Izod/Lacoste with the alligator logo.
Factual error: Shatter enables a hologram of the International Space Station from her wrist. The movie takes place in 1987 and the I.S.S. was not even started to be launched until 1998.
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