This phenomenon (side tone) occurs when you add the microphone level to the speaker output control panel or increase the microphone volume level - see Figure 1.

Solutions:

Some VoIP apllications are sensitive to this sidetone and generating unwanted behavior. Trough your speaker control control panel via "Options > Properties" (see Figure 2) it is possible to ensure no unwanted side tone. And here you must mute your playback of microphone of the output (see figure 3) which does not affect the microphone input level (see Figure 4)

Figure 1 (playback):

Figure 2 (playback):

Figure 3 (playback):

Figure 4 (microphone):