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I've found a solution that doesn't work by me:

audio - Monitoring the microphone level with a command line tool in Linux - Super User
https://superuser.com/questions/306701/monitoring-the-microphone-level-with-a-command-line-tool-in-linux

The problem is that they are using Maximum amplitude to detect sound. However its value is always the same by me, no matter whether the recorded audio contains only silence or some sounds. For example:

10 sec of silence (Can be downloaded here: http://denis-aristov.ucoz.com/en/test-mic-silence.wav ):

$ arecord -f S16_LE -D hw:2,0 -d 10 /tmp/test-mic-silence.wav



$ sox -t .wav /tmp/test-mic-silence.wav -n stat

Samples read:             80000

Length (seconds):     10.000000

Scaled by:         2147483647.0

Maximum amplitude:     0.999969

Minimum amplitude:    -1.000000

Midline amplitude:    -0.000015

Mean    norm:          0.202792

Mean    amplitude:     0.009146

RMS     amplitude:     0.349978

Maximum delta:         0.913849

Minimum delta:         0.000000

Mean    delta:         0.001061

RMS     delta:         0.005564

Rough   frequency:           20

Volume adjustment:        1.000

10 sec with some sounds (Can be downloaded here: http://denis-aristov.ucoz.com/en/test-mic-sounds.wav ):

$ arecord -f S16_LE -D hw:2,0 -d 10 /tmp/test-mic-sounds.wav



$ sox -t .wav /tmp/test-mic-sounds.wav -n stat

Samples read:             80000

Length (seconds):     10.000000

Scaled by:         2147483647.0

Maximum amplitude:     0.999969

Minimum amplitude:    -1.000000

Midline amplitude:    -0.000015

Mean    norm:          0.185012

Mean    amplitude:     0.010225

RMS     amplitude:     0.334286

Maximum delta:         1.999969

Minimum delta:         0.000000

Mean    delta:         0.006213

RMS     delta:         0.057844

Rough   frequency:          220

Volume adjustment:        1.000

What is the difference? What values to use for sound detection? Or do I have to set something up because something works wrong?

I've just used another computer (a notebook with a built-in microphone). I've recorded two WMA files (with and without sounds) using Windows "Sound Recorder". Converted them to WAV files using audacity and got the following outputs. Maximum amplitudes differ this time:

With sounds:

$ sox -t .wav /tmp/mic-sounds.wav -n stat

Samples read:            581632

Length (seconds):      6.594467

Scaled by:         2147483647.0

Maximum amplitude:     0.999969

Minimum amplitude:    -1.000000

Midline amplitude:    -0.000015

Mean    norm:          0.013987

Mean    amplitude:     0.000062

RMS     amplitude:     0.065573

Maximum delta:         1.999969

Minimum delta:         0.000000

Mean    delta:         0.011242

RMS     delta:         0.047009

Rough   frequency:         5031

Volume adjustment:        1.000

Without sounds:

$ sox -t .wav /tmp/mic-silence.wav -n stat

Samples read:            372736

Length (seconds):      4.226032

Scaled by:         2147483647.0

Maximum amplitude:     0.029022

Minimum amplitude:    -0.029114

Midline amplitude:    -0.000046

Mean    norm:          0.005082

Mean    amplitude:    -0.000053

RMS     amplitude:     0.006480

Maximum delta:         0.030487

Minimum delta:         0.000000

Mean    delta:         0.005815

RMS     delta:         0.007285

Rough   frequency:         7891

Volume adjustment:       34.348

May it be an indication that there are some problems with the microphone on another computer?

(Answer based on various comments, as this method seems to be acceptable, and comments are not guaranteed to stay.)

Look at the first recording ("10 secs of silence") in an audio editor, e.g. audacity. You'll see a DC (very low frequency) component when the level goes from 1 at 0 secs to -1 at 1 secs to 0.5 at 1.5 secs, and then falls down to near zero near the end. Did you plug in the mic during that time? If yes, you need to wait ca. 10 seconds before the amplitude settles, then measure. If not, you need to filter out the DC (direct current, that is constant voltage offset) component somehow. sox has several filters you can try.

You can use the sox filters from a shell script without problems. Try e.g. highpass 100, that filters out most of it except for the initial jump.

If filtering out DC components is too much effort, you can also ignore the initial part, and use the remaining part as it is.