#8 is a great concept. I like the headphones idea, but the center area feels like it's lacking sizzle. Can you play with some different ideas and see what you can come up with? Maybe some color in there? It's ok if the headphones become the secondary focal point... It might look cool if the center grabs your attention first. Thanks!
I really like what you've done with #29. I think you may be onto something there! I'm wondering if there's anything we can do to make the M's stand out even more? With regards to your question about being simple or more complex, I'd be curious to see what you would do if I said more complex. I do like where you are going with it!!!
#31 is a good concept, but I think the colors are too dark to see at first glance. You might try using the actual traditional meter colors... like green, yellow, and then red at the top to make it look like a digital meter display or something.
#30 is OK, but I definitely like #29 better so I say we stick down that road.