Comic Book Questions Answered - where I answer whatever questions you folks might have about comic books (feel free to e-mail questions to me at email@example.com). Here is a link to an archive of all the past questions that have been answered so far.
A few different people have asked me this question over the years. In the past, we've examined WHEN Deadpool started talking in yellow word balloons, but we've never explained WHY he talks in yellow balloons, except, of course, for the general "because he sounds weird" explanation that is why all characters with weird word balloons have for why they have weird word balloons (in other words, if you have a different type of word balloon than everyone else, the default explanation is that you sound different from everyone else). So what does Deadpool sound like, then (in the comics, that is - in the movies, he sounds like Ryan Reynolds)?
There are two notable references to what Deadpool's voice sounds like. The first came in Deadpool #14, the classic issue where we learn about Blind Al's relationship with Deadpool and...well...Deadpool comes out of it sounding like a total jerk, so I'll gloss over how disturbing Al's story about Deadpool is, except to note that she says the following about Deadpool's voice:
In Cable and Deadpool #12, Deadpool co-creator Fabian Nicieza (who presumably has an excellent idea of how Deadpool was meant to sound like when introduced) has Cable describe Deadpool's voice thusly...
So it sounds like the yellow word balloons denote that Deadpool has a raspy, gravelly voice.
EDITED TO ADD: Reader Nate M. stopped by to note that Fabian Nicieza recently cited 1990s Denis Leary for what he personally heard when Deadpool talked. Leary definitely fits with the aforementioned description.
If anyone else has a comic book related question, feel free to drop me a line at firstname.lastname@example.org