Hmm, yeah, that's true actually.....although, as I've played the game Transformers: War for Cypertron, Soundwave has keep his form in there whilst he was on Cybertron. Lazerbear, Rumble and Frenzy, that were in his boss battle, kept their original forms as well.
Man, it is rather confusing now....as on Cybertron, he seems to have been able to keep his alt form....although all playable characters had an alt form of either a tank, car, truck or jet and Soundwave got to have a truck as his alt form there.
So really, in that manner of speaking, it is a rathger tough one to figure out....but it does seem like both Autobots and Decepticons, since they were from Cybertron, have advanced technology as it is.....so really, I would imagine that Soundwave, at this point, would still be able to obtain his casette form somehow XD