Basically, it depends on the circumstances of the case. For guidance, I would look at the following case:
CLICK HERE. In this case, the Court effectively stated that cordless phones, while they do broadcast radio signals, give the user a reasonable expectation of privacy. People think that they are just like other phones.
The circumstances would be very similar to those of a wireless microphone, unless there was something about the use of the microphone that would let the user know that other people could easily listen in.
Although the case could go either way depending on the exact circumstances and use, it would be dangerous to listen in to such conversations as the potential for criminal liability would be great.
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DISCLAIMER - This answer is not legal advice. No attorney-client relationship is formed. The information provided is for educational purposes only. For specific advice regarding your situation, please consult a qualified attorney in your area.