The microphone socket is a 3.5 mm TRS type (three poles), it provides a bias current of 2.6 V on both the tip and the ring, so it is advisable, as reported in the device manual, to NOT use a simple two poles mono jack (TS type), because it will short-circuit the bias current on the ring with the ground on the sleeve.
The device is detected by the Linux Kernel 4.9.0 in this way:
usb 2-1.3: new full-speed USB device number 5 using ehci-pci usb 2-1.3: New USB device found, idVendor=0d8c, idProduct=0024 usb 2-1.3: New USB device strings: Mfr=1, Product=2, SerialNumber=0 usb 2-1.3: Product: USB Advanced Audio Device usb 2-1.3: Manufacturer: C-Media Electronics Inc. input: C-Media Electronics Inc. USB Advanced Audio Device as /devices/pci0000:00/0000:00:1d.0 /usb2/2-1/2-1.3/2-1.3:1.3/0003:0D8C:0024.0004/input/input18 hid-generic 0003:0D8C:0024.0004: input,hidraw2: USB HID v1.11 Device [C-Media Electronics Inc. USB Advanced Audio Device] on usb-0000:00:1d.0-1.3/input3
Launching alsamixer and hitting F6 you can select the USB Advanced Audio Device sound card. Hit F4 to view the only one capture device, i.e. the microphone input line. Press Space to enable/disable the capture and Up/Down arrows to increase/decrease the gain.
It seems that the best setting for audio recording through the mic, is to keep the capture mic gain to 100:
I use the PulseAudio subsystem above the ALSA modules (this is the preferred method in modern GNU/Linux distro). In this case the volume is controlled via the graphics app pavucontrol or on the command line using the pacmd tool. The PulseAudio recording volume level should be between the base volume (value 20724) and the 100% (value 65536).
Here are the commands to start an audio recording using a shell script:
#!/bin/sh SOURCE='alsa_input.usb-C-Media_Electronics_Inc._USB_Advanced_Audio_Device-00.analog-stereo' PORT='analog-input-mic' VOLUME=65536 pacmd set-default-source "$SOURCE" pacmd set-source-port "$SOURCE" "$PORT" pacmd set-source-volume "$SOURCE" "$VOLUME" pacmd set-source-mute "$SOURCE" 0 parecord --verbose --device="$SOURCE" --channels=1 --fix-rate foobar.wav
WARNING: The audio files produced by this USB adapter do not cover the entire 16-bit range; the values returned (converted into float dB) extend from -0.5 to +0.5, i.e. exactly the half of what expected (-1.0 to +1.0). Above that values, the audio is clipped. You can view this in the pavucontrol monitor: it does not matter how loud you speak into the microphone, the volume meter bar never reaches the full scale. So it is advisable eventually to lower the volume (e.g. to 75% or 50%) to avoid clipping, and to proceed with post-processing software amplification.