Soundwave and Megatron first met in the pits of Kaon, where they fought as gladiators. When they were matched against each other, Soundwave became noteworthy for being the closest to ever come to beating Megatron even though he still lost. The match forged a bond of mutual respect between them. Megatron himself has never questioned Soundwave's loyalty, and clearly has full confidence in him, having shown him favor multiple times. Soundwave himself shows nothing less than total devotion to Megatron, with unquestioning obedience, and always complies with Megatron's orders, as well as alerting him to possible traitors or signs of disloyalty. In Robots in Disguise, Soundwave attempted to summon back Megatron twice but foiled by the Autobots.
Laserbeak is Soundwave's only remaining Mini-Con following the events on Cybertron. He is mostly used for scouting missions. Laserbeak can zap and knockout opponents. When his live footage had crashed, Soundwave didn't have much of a reaction, but then again, he never does. Even so, he sensed when Laserbeak was in danger, and passed up the chance of killing Wheeljack to save his drone.
Soundwave and Starscream have interacted in ways that suggest Starscream fears the silent Communications Chief, as Soundwave has kept a close watch on Starscream, constantly being an obstacle in Starscream's schemes, which Starscream's irritably acknowledged to Knock Out once before. Soundwave clearly does not trust Starscream, seeing as how he sent Laserbeak to make sure Starscream brought the critical Megatron back to the Nemesis. They would be seen to hang out or chat (which Starscream is the only one speaking) in some occasions.
There is not much interaction between the two Decepticons, since Soundwave just provides portals for him when necessary. Soundwave did arrive to assist Shockwave in battle in Project Predacon (episode). Both cons worked well together to take down Optimus Prime, Bumblebee and Smokescreen and securing the Predacon bone.
When Airachnid attempted to take control over the Nemesis as Megatron was out somewhere, Soundwave and his pet Laserbeak took her down without any hesitation and forced her to stand down, which she yielded to Soundwave. Airachnid has even attempted to assassinate Soundwave with her armada in Thirst as revenge but failed as Soundwave managed to teleport them all to Cybertron's moons, which made Airachnid scream in rage.