Question: Why does Agent Tucker not appear in this film?
Question: Why did Arthur need to go to Atlantis to meet Vulko? We see Vulko coming to the surface multiple times, would it not have been easier to meet Arthur on land?
Answer: Because what they're looking for is in the ocean. The maps, the ancient texts and markers leading to the trident. It would seem odd to search for Arthur on land then return to the ocean.
Question: Why didn't Frank Welker reprise his role as the voice of Soundwave for this film?
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.
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.
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.
Answer: Because, it was time for a comeback, so they tasked Bough to complete the team.