Speech impediment
Chris has several problems that may stem from a Southern accent and dialect along with a genuine speech problem. It has appeared to worsen over time due to the fact that he spoke clearly and normally in his older videos, while he frequently stutters in the more recent ones. This could possibly be attributed to "stress" from dealing with the trolls.
His problems in verbal communication most likely stem from either the extended period of "muteness" that he was under, purportedly lasting from the age of one and a half years old to seven or his autism, in which speech difficulties are very common. One should note that what he types on the internet generally tends to be significantly more coherent and grammatically correct than how he speaks. This may indicate that while writing, he has more freedom to determine what he says. Given the various, glaring errors in the Sonichu comics, you can guess just how coherent his speech is.
Classes
Non-Impediment Words
Many of Chris-chan's abbreviations are common in Southern accents and speech in general: "Y'all," "'em," "gee-tar," "givin'," and so on are uttered by many people who aren't Chris. These may sound strange if you're not familiar with southern accents, but if Chris's only verbal problem was dropping the occasional g or l, there wouldn't be a "speech impediment" entry.
He also grossly mispronounces names, e.g. "Fills-eye-mez" and "Mayamoto", but this is easily explained as Chris being subliterate and not knowing how to pronounce non-English names.
Likely Impediments
Chris's diction is very staggered. He fills up dead space with "Um"s and "uh"s, or, when attempting to be sweet/romantic, "hmmmm"s, and further likes to pause and take deep breaths to break up sentences. Deep breaths and pauses are a training method for folk with speech impediments, so we know that Chris has some difficulty there. Especially with all the other crap that makes him so hard to understand.
Chris can't seem to pronounce the letter "X" very well, pronouncing it more like the German "Esszett," or double-S (ß). Thus we have Chris complaining about how homosessuals are wearing Asse body spray. Besides that, he tends to mash phrases into lumpy, assorted-candies-melted-together blobs that sound sort of like English while also sounding like his tongue is just flopping around in his mouth without actually communicating anything. Could you guess that "BUHbyenose" is meant to be "but if anybody knows"? Another fan favorite is "Ensykapeia Dramakka paysh". Even worse are when he mispronounces words in ways that veer wa-a-ay out of the "southern accent" excuse zone. Southerners cut off a lot of g's, but they're not precisely known for mixing up r's and w's as with "rell-wested" (well-rested) and "rishes" (wishes). At some point, trying to find a pattern in how Chris mispronounces words becomes futile, adding consonants to some words and taking them away from others. It's safe to say these are full-blown impediment issues. Combine all this with a constant stammer and you have the reason why there are so many videos without transcripts.
Slang and Word Phobias
In the case of words like "da," Chris might be trying to take a pass at using slang to sound casual and relaxed, and winding up sounding like a marketing executive trying desperately to sound casual and relaxed to his target audience. Others, including famous CWC-isms like "tobacky" and "gaybian," are the result of a different problem altogether: Chris is a wussy manchild who can't even speak the name of objects or concepts he finds repulsive. This isn't a speech impediment per se, but it's quite indicative of how immature he is. While it isn't technically a speech impediment, though, these CWC-isms combine with his existing speech problems and his accent to form what you might call logorrhoea, literally "word flux" or, more to the point, verbal diarrhea: a constant stream of almost-words that try to mean something and Fail, hard-style.