I don't know if there's been a formal audit, but they've published an overview of their cryptographic methods and practices [1].

They've also open-sourced KBFS itself and the `keybase` client [2][3], but again, not sure on the audit.

[1]: https://keybase.io/docs/crypto/overview

[2]: https://github.com/keybase/kbfs

[3]: https://github.com/keybase/client