· Can I use software to apply a "relative volume" offset to each track so they can sound equal?
No, at least not with good results. Take our "pop vs. classical" example:
If you applied a positive volume offset (increase) to the classical piece, then the peaks would distort terribly and maybe even ruin your speakers. If you applied a negative volume offset (decrease) to the pop piece, you would lose any subtle details in the quiet parts.
The only workable solution is to apply additional dynamic range compression at playback time.