There are several ways to adjust levels in Audacity. If you adjusted gain between songs during the original recording, it is probably easiest to select the data and use Amplify. If you adjusted the gain during a song, then I'd use the envelope tool to try and match the rate of level change. Otherwise, you will have a sudden change in level which will be quite noticeable.
And here is some unsolicited advice: Don't change levels during recording unless you are seeing a lot of clipping during the music, or the peaks are way low, maybe -20 dB or more. The great thing about digital is for most recorders with most music in most venues, if you get the levels within the same county you have enough S/N so that the recorder and pre-amp noise will be masked by the music and or ambient noise. This was a hard lesson for me to learn having grown up with RtR where if the stars aligned I'd get maybe 60dB S/N. Needless to say, setting proper levels during the recording were very important in the bad old days.