Some irregular nouns change their vowel sound to form the plural?

That's correct! Many irregular nouns in English change their vowel sound to form the plural. Here are a few examples:

* Foot -> Feet (the vowel changes from "oo" to "ee")

* Tooth -> Teeth (the vowel changes from "oo" to "ee")

* Goose -> Geese (the vowel changes from "oo" to "ee")

* Man -> Men (the vowel changes from "a" to "e")

* Woman -> Women (the vowel changes from "o" to "e")

* Mouse -> Mice (the vowel changes from "ou" to "i")

These are just a few examples, and there are many more irregular nouns that follow this pattern.

It's important to note that these vowel changes aren't always predictable and often need to be memorized.