You can adjust audio in FCP. This is called mixing. On the bottom of the timeline, on the left are what looks like two mountain peaks. click that and you have lines on the clips you can drag up or down. Or you can have the Audio Mixer open and mix audio there. Yes, it will be relative to the original sound...if you make it louder the original sound gets louder...I don't know what you mean by that. You can't just say "I want this audio to be -6db." You adjust it until it hits that point on the VU meters.
You can do this with FCP, or send to Sound Track Pro.
Shane