1) Get the HW input device:

$ arecord -l
**** List of CAPTURE Hardware Devices ****
card 0: xxx [XXX], device 0: USB Audio [USB Audio]
...
card 5: c920 [HD Pro Webcam C920], device 0: USB Audio [USB Audio]
...
card 6: PCH [HDA Intel PCH], device 0: ALC221 Analog [ALC221 Analog]
...

Let's suppose I want to capture the input from HD Pro Webcam C920. In this case it would be hw:5,0 (5:card, 0:device)
```

2) Monitor input signal:

```
$ arecord --device hw:5,0 -f cd --vumeter=stereo /dev/null
Recording WAVE '/dev/null' : Signed 16 bit Little Endian, Rate 44100 Hz, Stereo
Warning: rate is not accurate (requested = 44100Hz, got = 32000Hz)
         please, try the plug plugin 
           +###################### 66%|73%#########################+