Richard Lovejoy's volume adjustment kernel is a realtime volume compressor. It increases the volume of quiet passages in music tracks, so that they are not buried in road noise.

It works by slowly increasing the gain of a track, and cutting back on the volume if one of the track's peaks would clip. For instance, during the quiet flutes at the beginning of Peter Gabriel's "Sledgehammer", the gain is increased so that they can be heard. When the loud horn section comes in after the flutes, the gain is cut back again so that the horns are at a normal volume.

Although it can't make "all songs play at the same perceived volume", it does help towards that goal. I find that it does help certain songs with a lower perceived volume sound a bit louder. It depends on the song and how it's produced.

___________
Tony Fabris