Technology can be extremely helpful for helping bridge gaps between learner needs. Consider creating a hashtag for your event folks can use at the end of questions they tweet, and then have the speaker or someone monitor the question thread. You could also have someone type out questions onto a blank slide as they’re asked so there is also a visual representation. Pollev.com can also be a good tool to group think the questions.
Make sure you test the sound levels ahead of time. Even if you have a mic set up, sometimes they are not fully functional, so this is crucial. Have another staff person in the audience keeping track of if anyone is signaling they can’t hear to communicate with your sound system operator.