"audience-free", which in my opinion should be hyphenated, means that no audience was present. The speaker is saying that these congressional sessions were conducted late at night with no audience present, with an implied criticism that the issues may therefore not have been properly debated or scrutinised.
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VorparIt means secret/private meetings. "audience free" means there is no audience.It's neither secret nor private. I have watched some of these sessions on CSPAN which continually broadcasts these after-hours congressional sessions. It's the perfect solution for late night insomniacs.