Chosen Solution
During an evening of mixed video recording and burst mode the sound stopped recording - just total silence. I only noticed a couple of days later when I got to review the footage. No settings had been adjusted during the evening other than swapping video/burst.
I contacted GoPro and they suggested to re-update firmware and send a sample video. Having examined the video they have accepted the camera as a return under warranty.
Were you using a new SD card? Is it one of the GoPro recommended cards: https://gopro.com/help/articles/Block/mi… You can also try a factory reset and firmware update.
