AudioLoudness can compute different loudness definitions. Units are mentioned in the reference page.
"RMSAmplitude" gives root mean square of the values and for a typical audio data being in -1 to 1 range, will return values in the 0 to 1 range.
See this:
a = ExampleData[{"Audio", "DogBark"}];
Show[AudioPlot[a, PlotLayout -> "Averaged"],
ListLinePlot[AudioLocalMeasurements[a, "RMSAmplitude"],
PlotStyle -> Red]]
