Lack of Intelligible Speech
From Homestar Runner Wiki
A few characters, as well as their respective counterparts in 1936 and 20X6, have an apparent lack of intelligible speech. However, the audience is usually able to understand the basics what these characters attempt to communicate, either through their actions and facial expressions, through other intelligible-speaking characters' reactions, or, rarely, through on-screen subtitles.
 Characters with a lack of intelligible speech
The Cheat speaks only in squeaks and animal-sounding grunts and yelps that occasionally resemble actual words. In cheat talk, after failing to get him to say something normal like "Douglas", it is suggested that the viewers have to learn how to speak "The Cheat". Subsequently, in an Easter egg of that same email, a short recording teaches a few phrases of the indecipherable language. The other characters, while able to understand The Cheat perfectly, can't speak more than a word or two in his language. Notably, although he only expresses himself using these "The Cheat noises", he is nevertheless able to produce or somehow generate the voices for all of his Powered by The Cheat animations.
The Cheat's Old-Timey counterpart, The Sneak, communicates only by wiggling his nose (except in Sickly Sam's Big Outing, where he sang the word "telegram" when delivering a telegram), and his 20X6 counterpart, Cheatball, has yet to say anything beyond his own name.
Pom Pom speaks through sounds resembling bubbles. Like The Cheat, every character shows a clear and definitive understanding of his speech. (This is occasionally used for humorous effect, when the audience is left to infer that Pom Pom said something outrageous or off-color by the other characters' reactions to him.) In A Jumping Jack Contest, Strong Bad attempts to pose as Pom Pom by, among other things, mimicking his speech: "Hey, give me that! Uh, I mean — bubble, bubble-dee-bubble. Dubble Bubble." Also, in senior prom, Homestar accepts the prom king award on Pom Pom's behalf, and in doing so, gargles Listerine in an attempt to imitate his friend.
Sherlock, the helicopter/cow from Strong Bad's Sweet Cuppin' Cakes, speaks only through indistinct mumbling. No other character has shown or demonstrated the ability to understand what Sherlock has ever "said", though Homestar imitates his speech in the 2009 Punkin Stencils update.
The Poopsmith has been noted as having taken a vow of silence, but is still able to communicate in some ways. An ironic example of this was when, in senior prom, The Poopsmith was the lead vocalist of All the King's Men!. Since, unlike other characters, The Poopsmith makes almost no vocal sounds at all, he seems to communicate mainly in the form of gestures such as shrugs of the shoulders and thumbs-up; additionally, he has communicated by holding up written signs. In different town, Strong Bad claimed that one thing he would change about his town is that "the Poopsmith, he could talk". However, every two hundred Strong Bad Emails, The Poopsmith breaks his vow of silence to sing, as shown in email thunder. When singing, the Poopsmith is voiced by John Linnell of They Might Be Giants.