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I recall reading about something like that, too. There are funny sounds, and non-funny sounds. For example, the word "mute" is hardly humourous in sound nor meaning, but change one letter and you get "cute" and suddenly, completely different.
Now that I think of it, in general, "softer" sounds are not as funny, whereas letters like K, C, P (which have a "sting" to them with the ee sound) are all over the words you've mentioned.
Monkeys can get old, though comedians still tend to lean on them. I once read something about the phonetics of funny that suggested that monkey was intuitively funnier because of the k sound. (The bare-toothed ee at the end is also a nice grimace-or-grin segue.) Other funny words with k sounds: kumquat, guacamole, Clem, Kadiddlehopper, schmuck, kookaburra, wocka-wocka. The ear does get tired of that sort of thing pretty quick, though.