I just spent over an hour on DJI Chat support for the Osmo.
It appears that the "Mute Fan" option is only for the X3 camera and not an option for the X5.
The fan noise is terrible when recording sound with the X5.
They said to email the mainland China DJI Support to address the...
Bought a £4 ($6 US) lapel mic off eBay and tested it. Before and after video below.
The inbuilt sound is simply unusable as far as I'm concerned. Which is ok by me as I would never use a camera's inbuilt mic to record sound, but you might not agree.