Roiland confirmed this in a Q&A with fans on Gizmodo while also explaining his process. During the recording process, Roiland first records all of his dialogue lines normally. He then marks the lines he wants to go back to do “burp.” Unfortunately for Roiland, he can’t burp on command. Instead, he must rely on a combination of low-calorie beer (Miller 64 specifically) and air to generate the necessary momentum.
As a result, “Rick and Morty” has real, high-quality burps, and no two of them ever sound quite the same. You just can’t get the same product by using sound effects.
Roiland’s comments aren’t surprising considering he actually got drunk to record a Rick and Morty episode before. During the fourth episode of Season 3 entitled Vindicators 3: The Return of Worldender, Roiland was slammed at Dan Harmon’s suggestion. The resulting video has garnered more than 29 million views on YouTube. Give it a try if you don’t mind strong language.