I'm pretty sure he is a seeker. He has wings and he flies. Doesn't use a broomstick to fly, though. Which is odd....
Kidding!
I have read many stories that say he is a seeker, though I don't understand because would he still be considered a grounder from his past episode appearances? And how do his wings stand all the stress if he is a seeker. But then again, he uses his arms to type and those are his wings. It's hard to say.
After reading a bit on Seekers. (The first paragraph of Seekers- Transformers Wiki) Soundwave would not be deemed a Seeker because he is not part of Starscream's trine. He does not have a body shape of Srarscream's nor a head like his. He is not a jet in g1, therefore he would not be a seeker.