This is where Soundwave returns in Stephen and the Scraplets
Optimus Prime: [narrating] After the death of Unicron and Tirek, things were the way they should be. That was until an old enemy we once faced struck back, with an army of metal eating monsters behind him. However, we were helped by the one they call Stephen the Rocket and he proved himself worthy of the skills of a Trainbot knight. And the first few lines that I told you... Is where our story is about to begin.
[We zoom in on Planet Earth as the title goes by. We then view the Shadowzone from a short distance. We peak over a cliff to find Soundwave trudging around in a trench. He suddenly sees a colony of Scraplets]
Soundwave: [thinks for a moment] [as Megatron] Good Plan! [uses a hypnosis device to hypnotise the Scraplets]
[The Scraplets gaze upon their new leader. Soundwave suddenly plays a recording of Knock Out's voice]
Soundwave: [as Knock Out] Must have revenge.