1. Make sure iPod volume is where you want it. Check
2. Go into iTunes and hit 'Volume Leveling' to even the volume of all your MP3's
3. Play audio through AUX port.
4. If your still using the stock head unit, adjust your Bass/Mid/Treble numbers to get a little bit more quality/volume
5. If you're using an aftermarket head unit, look for some kind of "Loud" feature, if there is one, turn that ON.
Sadly all CD's will play louder/have better quality than any MP3 player. It's just part of the problem. An Aftermarket Head Unit won't make this as pronounced, or if you have an after market amplifier going to your speakers as it will boost the signals power level. But with the stock system, you're kinda limited to what it gets. This is why I don't use CD's in it and only use my iPod.
Hope that helps.
