Answer:
The mice's squeaking.
Step-by-step explanation:
In simplest terms, usually using the noun and adding an apostrophe will make it possessive.
Ex.
Not Possessive: The writings of Ernest Hemingway are complex.
Possessive: Ernest Hemingway's writings are complex.
(Sorry for the not so great explanation, it's kinda just something to get used to, at least in my opinion)