There's s little app in the Marketplace that could be of use to many: Media Volume Settings by Andrea Sabbatini. Version 1.2 just landed and it comes in two versions: trial (fully functional, free but with ads) and paid ($0.99, no ads).
The app is simple enough: on Windows Phone 7, when you go to play music, videos or games, the "media volume" is separate from system volume. That's why in some games, like my favorite Decimation X2, they have a separate slider for "music" which you can lower or raise. This aps gives you global control over that setting, allowing you to choose a default level. This is great in case you want to sneak a game in somewhere but don't want the music to blare through the speaker, announcing your universal boredom with the people around you.
The app has just five settings: Mute, Soft, Middle, Loud or Max plus technically one more--custom, whereby you adjust the slider to your liking. We've tried it on a few devices here and it seems to work quite well. Seeing as you can have it for free, it certainly can't hurt to give it a go. Grab it here in the Marketplace.