Strange new audio problem with GH3

[Stab]

I shoot weddings with the GH3. Last wedding I encoutered a new problem. When I record audio with an external microphone, there is no problem.

But when I record audio with the onboard mics, I've noticed something strange. The first few seconds of the clip sound normal, but then suddenly it's like there is an EQ applied and the bass / low mids almost completely disappears. Also, it's a bit like it shifts from stereo to mono.... In other words, the audio suddenly sounds very different.

This seemed to happen on many but not all clips in the evening after I unplugged the microphone. The music wasn't very loud so that's why I'm surprised that this happened. It doesn't really sound like it's clipping. 

Any idea on what this might be? 

[leeys]

Never happened on my GH3. Does percussive maintenance while recording solve the issue temporarily?

[ac6000cw]

The effect sounds like the auto wind-cut filter may be kicking in - make sure it's disabled in the menus.

[Stab]

The effect sounds like the auto wind-cut filter may be kicking in - make sure it's disabled in the menus.

​I think this might be it!!!

It very much sounds like that though. I didn't even think about this because I always set that to 'off'. But, my camera has been repaired 4 weeks ago and had a factory reset. So yep, it was on 'auto' again. I turned it off now. I'm not going to test it now because I can't really mimic the specific cirumstances but I'm pretty sure this has to be it. 

Thanks a lot! Smart thinking.