I believe one-party state argument applies to only two or more private parties and not a conversation between a private party and the authority or corporation where no crime is committed. For example, when you or I make a phone call to our bank or some place of business, there will be a recording stating that this called may be recorded. That is a warning and we have the right to continue with the call and that implies we consent to the recording. There is none of that at the airport or bank and I have never seen a bank or airport video with sound.

If they want to record sound, just inform the private citizens verbally and/or in a manner clearly understandable. That is all many of us ask for. Now if we don't know if we're being audio-recorded and the system is setup to do run all day the way video recording is, then there is no way for us to voice objection to it. Just like the video recording, by seeing it and not object to it, we are consenting to being record on video. But we don't see sound and need to be told that we are being monitored. As stated earlier, one little sign hidden somewhere is not enough. If one doesn't see the warning sign, one has a reasonable expectation to believe it doesn't exist. For example, you can smoke at places that don't have "No Smoking Allowed" sign(s) because there is no instruction prohibiting you from smoking.