AO3's new ToS specifically mentions podfic under their II.D. Copyright Infringement section. In short, AO3 considers podfic to be derivative and not transformative. Because of this, you must have permission to post a podfic of someone else's work.
This means you can respectfully ask someone for permission, use the "Podfic Welcome" tag on AO3, or find an author who has a Blanket Permission Statement (BP). This is a statement, usually located on the author's profile page, that tells others exactly what they can and can't do with their fanfics. Note, some authors do have blanket no podfic policies.
Click in the links below or scroll down to learn more about what each of these resources has to offer.
The Fanlore article on the origins of asking permission before recording
The Fanlore article on what BP is
An FAQ for fanfic authors on what BP is and why they should consider having a statement
An introduction to permission statements for authors and other fan creators. Why you should leave it and how.
An interactive tool to help you build your own BP statement.
Click here to learn more about it.
The Fanlore article explaining what Dear Author letter are and their origins.
A database of authors with BP statements
A browser extension that highlights the names of authors with BP
A post on Amplification community where authors commented if they were okay to have their works recorded