A couple months ago, my Xbox 360 (original, not slim) started having audio problems. When watching Netflix movies, whenever the audio in the movie gets louder (people yelling, loud action scenes, etc.), the audio coming out of the console gets very quiet, so that I can barely hear it. As soon as the sound in the movie returns to lower levels (normal dialog, etc.), the audio coming from the console returns to normal so I can hear it again. I think it does the same thing in games (I've seemed to notice the audio getting quiet in games occasionally), but I'm not sure. It's mostly in Netflix that I notice it.
My girlfriend, watching the same movie on her Xbox (via Netflix Party), doesn't have this problem, so it's definitely something with my console.
I thought maybe it was my TV, but I connected the audio from the Xbox cable directly to some media speakers, and it does the same thing. I'm using the component cable (with the red, green, blue video plugs and red and white audio plugs). I don't know if makes any difference, but in the console audio settings, I have Digital Output set to Digital Stereo.