Can you show the Video Settings tab?
I have found I get less complaints if I save my video in 540p or 720p. I never upload 1080p videos.
Most of my audience is looking at the webinar on a laptop computer or tablet. I've tried various video formats and found that 540p works fine for all of our videos. If I thought my audience was viewing these videos on a 72" 4K TV screen, then I might start uploading 720p videos.
540p is a smaller file size than 720p, and thus seems to be able to fly through the Internet faster, with better success than a higher-resolution file. When played during a webinar, the video is downloaded from the GoTo Cloud directly to each Attendee computer.
The Attendee's computer has a lot to do with whether they hear the video sound or even see the video at all. I always have a second computer running during a webinar to see what the audience sees. Most times the video plays on that computer up to 15 seconds behind the video showing on my Organizer computer. Sometimes the video does not even play on that computer. My guess is it is a bandwidth problem that can't handle the data stream for a video, which is more than regular talking and much more then just showing a PowerPoint slide.
Also, MP4 is just an envelope that holds compressed video information. There are numerous compression rates and different file formats that can all be used to insert a video into an MP4 envelope. Not all computers or other devices can handle the same MP4 video file. There are some MP4 standards that most computers can handle and some variations that only work on certain devices. Exporting from Camtasia should create a standard that will play back on most devices.
The MP4 standard file format reminds us that there are enough "standards" out there so everyone can have their own. (which defeats the purpose of having a standard in the first place)