Mickey Mouse's entry into the public domain comes with significant caveats. While the Mickey Mouse who appears in Steamboat Willie (and other media published in 1928 or earlier) is free to use, there's established precedent that specific elements of a character which appear exclusively in later works which still fall under copyright may be protected, if sufficiently distinctive.
(This is the basis of, e.g., the infamous "Sherlock Holmes can't respect women" lawsuit: the Doyle estate, which at the time owned only a tiny handful of the latest-written stories, the others having already fallen into the public domain, argued that specific personality traits which Holmes exhibits only in those later stories are sufficiently distinctive as to be the valid subject of an infringement claim.)
With respect to various elements of Mickey's visual design, such as his red shorts and signature gloves, the matter is clear: just don't use those for another few years. However, there's another thing Mickey's public domain iterations don't exhibit: speech.
The present consensus among copyright scholars seems to be that "a character speaking" is not sufficiently distinctive as to qualify for protection, but the vocal characterisation with which Mickey Mouse is famously associated may so qualify. So, if you want to be scrupulously safe, you can have him talk, but not in that exact specific voice.
Which raises a fun question: what voice would you give him? Wrong answers only.







