As mentioned by another poster, I find that my Roku outputs audio (on all channels) at a much higher gain than my other devices (cable box, blue-ray, etc.). I really have to turn the tv volume down when switching to the Roku. And no, my TV doesn't have indivudual gain adjustments on inputs.
Edit: I have a Panasonic 50U series plasma --one model older than the poster above-- and I don't have the equalization settings he does.
2nd Edit: Disregard first edit, lol. I went back in to the TV settings and found that while there is only one "master" audio section in the menu, the settings in this one section do change dynamically depending upon which source you are using. In other words, it's not like I have an audio menu for "HDMI" and another for "Component", etc. But when I switch between sources, whatever settings I had set for HDMI and Component (I have one HDMI in) change. I happened to notice that my Component settings were being boosted by 6 on its scale, a pretty hefty increase. I guess I must have had the gain boosted at some prior time with a different device attached. So much for my memory! I have the Roku on my Component in. I brought it back down to unity gain and now the audio level is consistent with my other source inputs.