We went to Japan last February for a vacation, I was able to take a lot of videos while there. I checked a few videos immediately upon returning home and everything seems fine; but just recently, I was going through all of the videos I took and realized that there are some videos without audio, I was using both the on-board mic and an external mic. I was wondering if there's any way to fix the issue without sending the Osmo anywhere?

I already sent DJI an email, here's hoping it's not hardware related.

Congratulations! You purchased an osmo.  

I also just retired from Japan and I also have many videos with missing audio or audio drops out part way through the video.  

What can you do?

1. It seems using the physical button sometimes results in audio failing to record.  Always use the button in the app.  This seems to have helped me.
2. Do you leave the fan off when recording? I had mine set off to avoid fan noise... but I think this causes over heating and the audio drops out as a result.  Always use an external microphone located away from the fan.  If possible use a 2nd audio recording device!

Yes unfortunately even if you are half serious with sound recording in your video, you have to get an external device like zoom h1 and record externally away from the noisy DJI. I also learned about this after buying the Osmo and found out the hard way.