That's a good question. Maybe he hacked a military network and got a picture of Bumblebee. It's difficult to say because, unlike the other robots, Laserbeak seems to be able to switch his alt-mode at will.
I think at one point, Jerry (ugh) mentioned that he saw several pictures of the Autobots with Sam in the background. So apparently, there ARE photos of the them floating around.
Maybe its a TFA concept where multipul people where the same body type and laserbeak just chose one of those
There shouldn't be question to this really. Soundwave is an information specialist, so clearly he knew who are the main autobots, and especially the one that killed his pet Ravage. Lazerbeak scanned BB because that might have been the autobot he disliked the most, but children might find cute.
i think he downloaded the pic of bee from the goverment and transform into him and beside Bee is cute that will attract the little girl