That might be a defect in such camera as model #s (c100 and c300). Or it might be that they both need to be updated to the latest version of firmware for their exact model, from canon's site not any other - and carefully! a firmware update done the least bit sloppily can ruin a good camera quite badly... and expensively.
Or, you might need to record audio on the same device that you record video.
Or, you might need to use better cameras period - e.g. the ones you said do fine Vs. the Zoom H1s, the canon 70D ones... to eliminate this problem.
I doubt it is a defective-camera issue though... they are both of one model series, and 2 other cameras from the same maker but a much different model series, are fine.
One last thought: are the cards you are shooting video onto, fast enough? if there is delay is saving data to them, it might just slow down the video just that much! fast memory cards are expensive for sure, but if you'd rather use the two cameras with the problem, there is the most likely solution to the problem if you must use them! I recommend that to your attention for sure.
- Does this answer your question in full? I hope so! but if you need other details, just Reply and we'll work on them further.