open discord, bottom left click on your user preferences icon, then select voice and video on the left choices.
make sure your input device is your microphone and not something else like a usb cam mic.
set you input volume to max. set input mode to push to talk. select a key you will press to talk (I use alt) then esc and try again
note in most groups you have to click on a voice channel on the left to be able to speak with others. or open a direct conversation with a person.