sox -t .wav "|arecord -d 2" -n stat

With -t .wav we specify we process the wav type, "|arecord -d 2" executes the arecord program for two seconds, -n outputs to the null file and with stat we specify we want statistics.

The output of this command, on my system with some background speech, is:

Recording WAVE 'stdin' : Unsigned 8 bit, Rate 8000 Hz, Mono

Samples read:             16000

Length (seconds):      2.000000

Scaled by:         2147483647.0

Maximum amplitude:     0.312500

Minimum amplitude:    -0.421875

Midline amplitude:    -0.054688

Mean    norm:          0.046831

Mean    amplitude:    -0.000044

RMS     amplitude:     0.068383

Maximum delta:         0.414063

Minimum delta:         0.000000

Mean    delta:         0.021912

RMS     delta:         0.036752

Rough   frequency:          684

Volume adjustment:        2.370

The maximum amplitude can then be extracted via:

grep -e "RMS.*amplitude" | tr -d ' ' | cut -d ':' -f 2

We grep for the line we want, use tr to trim away the space characters and then cut it by the : character and take the second part which gives us 0.068383 in this example. As suggested by comments, RMS is a better measure of energy than maximum amplitude.

You can finally use bc on the result to compare floating-point values from the command-line:

if (( $(echo "$value > $threshold" | bc -l) )) ; # ... 

If you build a loop (see Bash examples) that calls sleep for 1 minute, tests the volume, and then repeats, you can leave it running in the background. The last step is to add it to the init scripts or service files (depending on your OS / distro), such that you do not even have to launch it manually.