It all comes down to Soundwave’s absence in TFilm1 (he was replaced with Blackout, Barricade and Frenzy, two of whom got killed). This leads into two definitely likely threads: either in ROTF he’d just arrived; or in DOTM he’d established his network on Earth in the 1970s and then departed to return at a later time.
I’m taking the first one. It makes things easier and needs no reference to tie-in comics. And the if Soundwave had been around in the 1970s there is a very high possibility that he’d have learned of Sector Seven and its subjects the AllSpark and Megatron (just as Blackout did in TFilm1), and would have busted them out eventually (or even rapidly, depending on what he’d learned).
But still the DOTM theory is possible. A little unlikely IMHO, but still possible.