Yes I talk to myself.
Yes, I tell other people how they should be driving.
Yes, I play music.
Yes, it records people talking.
But when all's said and done - it's YOUR car.
Everyone who gets in my car is well aware of my dashcam - just the same as everyone who gets on my bus.
It's really easy to mute parts of audio during a clip (I use PowerDirector8), it's also easy to mute the whole clip or even split the audio from the video.
However, I do feel it is important to record audio.
You can hear my indicator - Might be important evidence in an insurance claim - they say you were indicating to turn into the road they pull out of, simply play the video & find an example of the indicator 'clicking' as soon as possible before the incident.
You can hear my horn - Again, might be important. someone claims they didn't see you & you didn't warn them...
You could hear my ABS kicking in - shows how hard I was braking?
You might hear panic in my voice at the possible impending impact - "Ohhhh CCcraaaap..." :-D
Someone does something stupid - maybe drives straight across your path? - the cams won't catch the polate but if you (or a passenger) see it, you could read it aloud! Might also be important in a hit & run.
I would far sooner have all the advantages & be able to edit out the disadvantages.
And as to 'private' conversations, yes, we've had some of those in the car - but it's easy enough to format the card.