The license file (as of 6450441) (in regards to audio files) does not specify a specific strong public domain clause or creative commons license:
```
Sound files are in the public domain. The rest of the code, and it's documentation,
is under the GNU GPL v3+.
```
While not strictly necessary for public domain audio works (at least, to my [ariana.giroux@gmail.com] knowledge and in Canada), this is not something that will hold up in court. It is best to include a Creative Commons license that would apply to the audio files. My [ariana.giroux@gmail.com] suggestion would be to utilize a CC BY creative commons license.
This license is extremely permissive, only requiring credit back to the original author via derivative and commercial applications. This is already required for the code base via the GPL, so including a similar requirement for the audio files is not a stretch.