Has Strongbox been audited for security?

Short answer: No.

Longer Answer: The database formats supported by Strongbox have been audited, and the source code for Strongbox is available freely online.

An audit on Strongbox source code has never been done, though it would be great to have that done, Strongbox is an independently developed and supported app..

Further Info:

Password Safe Format Audit

A full security audit of the Password Safe design can be found here:

https://www.cs.ox.ac.uk/files/6487/pwvault.pdf

Strongbox is a client built for the Password Safe file format, and is compatible with any other password safe applications. This format was designed by renowned security expert Bruce Schneier. A more general answer to the question can be found here:

https://security.stackexchange.com/questions/11192/how-secure-are-the-password-files-used-by-password-safe-and-password-gorilla

Source Code

You can also find all the code for Strongbox online at:

https://github.com/mmcguill/StrongBox