When you are in Audacity and play a track, the meter levels show you a bunch of things, in particular, the average audio level (that's the light green portion of the meter) and the peak audio level (that's the dark green portion of the meter). The average audio level and the peak level can also be seen when you switch the wave display from "waveform" to "waveform (DB)" - click on the arrow next to the name of the wave to make the switch. The light blue is the average audio level and the dark blue is the peak. It kinda gives you an idea but it's far from ideal, right?
What you really want is some kind of wave analyzer that can spit out the average audio level for the whole track. Well, low and behold, there's actually a plugin for Audacity that does just that. You can get it here: "Wave Stats" plug-in for Audacity. There's another plugin called "Wave Info" but it's the same thing, it just outputs the values in a format that you can more easily copy and paste. "Wave Stats" gives you the whole important although somewhat controversial average audio level known as the RMS (root mean square) average audio level (it kinda measures loudness). "Wave Stats" also gives the A-weighted RMS average audio level. A-weighting takes into account the big difference in terms of perceived loudness between bass (low pitched) and treble (high pitched) sounds of the same power. Indeed, bass never sounds as loud as treble because we, humans, hear high frequency sounds much better than low frequency ones. In a nut shell, A-weighting gives lower weights to low frequency sounds to account for the human ear.
Since a photo speaks a thousand words, a video must speak a million of them. Here's a little video I made about how to get the average audio level of a wave track in Audacity.
For now, you can only process the first 30 seconds of a selection (by default, it's the first 10). I am sure that, by the time you read this, you'll be able to process a whole tune, although it could be just wishful thinking.