I doubt any of this is going to help you… Human ears and a human brain are probably the best tools for the job.
How can I determine the average value of the volume of the song?
The “most reliable” way with free software would probably be to use the [u]SoX[/u] stats command. SoX is a command line program and RMS is a kind of average level. (I believe Adobe Audition can do various statistics but it’s not free.)
You can also try Audacity’s Analyze → Contrast (but I think it only works on mono files) or there is a plug-in called [u]Wave Stats[/u] that may work (but it may have trouble analyzing a whole song).
And the frequency range in which the people sing?
Wikipedia lists [u]vocal ranges[/u] from bass to soprano as musical note ranges, and you can cross-reference those against a [u]Note/Frequency Chart[/u]. (Roughly 100Hz to 1000Hz.)
Note that those are the fundamental frequencies and there are overtones & harmonics above that. “T” and “S” sounds may extend up to around 10KHz. (Speech or singing sounds muffled if you filter-out the frequencies above 1000 Hz.) And of course, there is a LOT of overlap with the musical instruments.