How to get audience audio in a small conference/training for 40-80 people?
Here's a video that covers everything you need to know about sound recording for video: http://nofilmschool.com/2015/08/24-sound-basics-will-help-you-lay-sturdy-foundation

If you don't have 18 minutes to spend, the specific answer to your question is that basically you cannot capture well the human voice when a mic is more than 3 feet away.  Even if you use a high-quality mic and a high-quality pre-amp, the quality of your audio will drop precipitously as you break the 3-foot rule.  (There are exceptions such as professional singers and actors hitting PZM stage mics, but even that is not great.)  

If you place mics around the room in such a way that nobody has to throw their voice more than 3 feet to reach a microphone, then you might have a solution.  But that might require 20-40 microphones, and an audio system capable of recording 20-40 inputs simultaneously.  Impractical.

Alternatively, you could have 3 mics, one for each zone of 15-30 people, which you can rapidly run to whichever audience member is trying to speak.  When they hold the mic (at least 1-2" from their mouth and at most 3 feet away), you should get fairly good results.  That would require an audio system that can record four tracks simultaneously (one for the podium mic and 3 for the audience mics), which is very simple nowadays.