I'm not sure this is exactly what you're looking for, but it sounds like it.
Go to the Settings app and under Sounds turn off "Change with Buttons" under Ringer And Alerts. The volume buttons will change the volume for music, games, and other apps, but ringtones, alarms and other alert sounds will still come through at full volume (or whatever you set the slider to.)
I have "change with buttons" enabled, and mine works fine. It only works if you're changing the volume while nothing is playing. If a Ringtone is playing, you can still change the volume, regardless of how you have the "change with buttons" set. The same is true with other sounds such as with games and music. I have the change with buttons toggled on. But if I lower the volume of a game's sound while it's playing, it doesn't affect my Ringtone/alert volume. It is a bit confusing.