Irresective of the rules of evidence, audio recording of another without their consent (an example would be an idiot "consenting" by leaving a voicemail message) is against California law. Your last sentence makes no difference to your argument.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.