Logitech LS21 recognized as 2.0 system - Windows 8.1 doesn't detect Subwoofer

The sub has a dial for controlling the thump, at the back - Lots of 2.1 systems connect like a 2.0 system, with a stereo cord going to the subwoofer, with a crossover sending out sound to the speakers. In general subwoofers are more 'felt' than heard, but yours is probably turned down.

If you have a 5.1 or better system the subwoofer has its own connector but here, the magic is entirely done on the speaker's inbuilt amplifier. While your system will 'see' this as a two speaker system, its not. Everything is working as designed.

All you need to do is to turn up this knob to taste

(image captured from tigerdirect's youtube channel)