Use the Gain Slider to add some gain to the level of the microphone. You typically have 2 options: WDM and MME. Clck for example over Hardware Input 1 and choose the desired device. To connect a microphone to Voicemeeter you can use a Hardware Input.
Typically Windows will provide a gain control for recording devices in the Levels tab of the Properties window in the Sound control panel, similar to this:
