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I always find it hilarious when people ask inherently spoilerish questions, yet simultaneously ask not to be spoiled. OP: How do you expect someone to answer the question "What's X character's role in the movie?" without revealing anything about the story and that character's place in it. Here's the best I can do: Spoiler Soundwave, from all reports and depictions (movie, novel, comics, etc...) does very little. He never comes to earth or adopts a terrestrial alt. mode, and he has minimal lines (in fact, only one in the novel). He never even transforms, he spends the entire film in his cybertronian alt. mode linked to a military satellite.
Ok, just got back from it. Spoilers, obviously. Heavy ones. Spoiler Soundwave is actually very important to the plot. He finds out where the cube shard is, finds out where Megatron is, dispatches Ravage to find the former. You see his head a few times, he gets a few lines, his voice is... not a demonic Megatron. Basically, it's his Soundwave voice without the modulation. So think Dr Claw with lots of bass and extra eerie. I liked it, but I think I'd have prefered the vocoder thingy. Oh! And Ravage launches from his chest. Which was great.
Spoiler No vocoder? The most iconic element of Soundwave (in animation at least) and it's not there? Bum.
Spoiler THATS IT he sounds exactly like Dr Claw...I don't think he even added extra bass "I'll get you next time Prime...NEXT TIME!!!" Makes sence too...Ravage = Madcat
You forgot to mention the bit where Megatron dies and Soundwave takes over leadership... Specially when he barks orders at Starscream with that fucken Frank Welker vocoder voice! Hellyes!
To try and keep it un-spoiler, all I will say is to imagine a see-saw. When we all first heard SW was in this the see-saw tipped to 'OMFG Awesome' All the reports of his diminished role (non-transforming, no vocoder etc) tipped the see-saw to 'lame'. This is good because Soundwave's role in the movie isn't as bad as you're now all expecting it to be. He doesn't come to earth or transform, but he's doing a pretty good job up there in orbit, so it wouldn't make sense for him to. They still could've done his voice properly though. Personally, I'd rather have had some random guy doing his voice with the vocoder effects rather than Frank Welker without them, but it hardly breaks the character.
Maybe they'll do the Vocoder for the next one? Who knows. It was Soundwavey without it, just not "classic." And I don't know why people are raging so much over Soundwave going to Earth and Transforming, I mean, I understand that you want to see him in robot mode, but come on, it wouldn't make any sense story-wise. He's doing such a good job up there, why is he gonna go down to Earth and risk getting killed for no good reason? Wait 'til next time where he has something approaching a logical reason to do it. In ROTF, it's a shame to say it, but it would have been pointless for him to go down to Earth. Spoiler Just be happy he didn't die.
Surely telling you Soundwaves role in the film is in itself a spoiler. Without revealing any spoilers, I can tell you Soundwave is in the film and he does stuff.
Damnit, why did you have to go and spoil it by saying and then . That's it , I'm not going to see this movie now!