For your Rio Car, there's Richard Lovejoy's Volume Adjustment Kernel. It is a dynamic range compressor that works in real time during playback by intercepting the audio stream and altering it before it's sent to the DSP. It makes the quiet parts louder. Its main purpose is to make the quiet parts of songs rise above the noise floor of the car environment while you are driving. But it also helps make songs sound like they are closer to equal in volume.

The volume adjustment code is available as part of Mark Lord's Hijack Kernel, under the heading 'Auto volume adjust' in its main menu. You have the ability to turn it on and off and set the degree of adjustment from this menu. See http://rtr.ca/empeg to get it.

Note that this changes the character of the way some songs sound. Depending on the song, it may be subtle or it may be overt. That's the nature of compression. If you decide you don't like it, you can turn it off.