I took a lot of videos at an air show recently. There were lots of obnoxiously loud speakers set up in the viewing area blasting music and commentary, as well as a lot of airplane noise. I was within 3 feet of one of the speakers and the audio turned out great!
I haven't tried just audio recording at a concert yet, and I'm sure that the sound pressure levels would be a lot higher. However, judging by the experience with my air show videos it seems the S6 would fare much better than the S4.
If you've got a good stereo at home, crank it up to 11 and try to record that. That should give you some idea of quality.
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