Voice chat disable in-game sound.

[Rylund]

Go into options and voice. Below the mic testing bar there is a box that says boost microphone gain. Uncheck the box and it seems to solve the problem.

Or try:
Right click your volume icon, go to playback devices, Communications tab, and check the lowest box for do nothing,

Sadly none of these solved this as my Gmod had Boost microphone gain off + I had it to do nothing already. 

Thanks for replying with possible answers.