Glad you were able to figure it out. I'm not aware of any other way to access the identity provider id from the response, but that does seem to solve your problem, correct?
Please feel free to file a feature request more clearly outlining your use case and proposed ideal solution.
It looks like the type of audiences is a set of strings, so I wouldn't expect any complex elements.
The docs say that this field is:
[a] list of the audiences for this SAML response. By default, the issuer or audience from the form are used.
This is not super clear to me, so I filed a PR against the docs to make it clearer: https://github.com/FusionAuth/fusionauth-site/pull/376