It may seem like the opposite way,but actually it was easy to find an alternate mode for Soundwave; Shockwave never adopted (and never needed) an earthly mode. So,if Soundwave is able to fly or work as an spy,all he has to do is show a hologram,somewhat similar to Blackout's fake pilot or just straightly call Rumble n' Frenzy to do a subterraneous hideout (stealing a base from any ultra-secret intelligence human organization also works).
So I guess it's time to tell already what this thing is and avoid more layers to preceed it.
This mode is a ̶D̶e̶c̶e̶p̶t̶i̶c̶o̶n̶ Cybertronian Sentry-Satellite,it hides the turret and the legs as a satellite,but can also show holographic images that share its size or are bigger.
The "Mouth" becomes Biped Soundwave's chest-screen,it can work as a screen,do energon cubes,suck energy from surroundings,display sounds (even mimic a few ones) but it has no offensive uses other than charge the turret or play extremely loud noises,of curse it can puke out Soundwave's mini(c)ons,and show holograms.
The thing is,that the turret can shoot both energy/energon blasts aswell as bullets,Soundwave is able to work as satellite,TV,Projector,scanner,boombox,radio,disco ball,airfan,microwave,sentry,security camera and a simple crab-like walking turret,all that at the same time in his biped or in this mode.
So what would be the reason for him to transform then?
The sentry mode is able to move faster,work more efficiently as satellite,and push the energon faster,but the turret is not as precise or quick at shooting stuff as when Soundwave gets into his callofduty-tier-sniper mode.
Notes: Blaster would have the similar mode but more focused in a energon processing machine,this one is for military purpose.
-The "Mouth" can also absorb solid matter in a similar,but less potent way to Devastator in ROTF,but it has totally nothing inside of it to crush that stuff,all the space there is used for the technical, intellectual,superior functions of Soundwave.
-Perceptor would also turn into a machine like this one,being able to work as thermometer,pluviometer,barometer,weighing scale,cash register,and a lot of other things that end in "er",but the standard use would be of curse,a microscope-telescope,Thermal Cannon
,Scanner,and many things,as long as it avoids the military theme.