Some months ago I tested one of the 11.X versions and had the same problem. Aaaand now I tested the latest 11.8 version - still the same problem.
I have PERFECT audio with 10.3 and HTML5. Sounds exactly as it should.
But with a later version of the Flash player, there's somehow "less" surround to the audio on videos and the volume is louder. This has nothing to do with sound stutter or any of that sort. It's all about the fact that the quality is just plain worse. It's like the sound is... um... Mono? When it should be stereo/surround. Again, it's perfect with 10.3 and HTML5.
Sooo... any ideas? I tested the latest version again after all those months because I kinda expected this problem to be gone. But no, it's still there.
I use Firefox by the way.
Edit: Perhaps it's after 11.2 version that this problem occurs. Figured it might be after reading another thread about something similar. Not sure though.