Audio Watermarking in MATLAB - How to choose frames to embed in?
Loudness is defined as human perception of volume. In general, there is a very good correspondence between the two. Therefore, you can often use one as a proxy for the other.

In this case, you can easily measure volume: the portions of the audio signal with the highest signal intensity (or volume) correspond to the parts with the greatest absolute value (or, if you prefer, square value). With a few assumptions, you can say that the loud sections are also the sections with high volume.