We hear you but you can't hear us.

We can hear you but you can't hear us.  This usually means your headphone/speaker settings are wrong.
Go to Discord settings (the gear icon) -> "Voice & Video" -> "Output Device"
Make sure "Output Device" is set to the right device.
If "Default" isn't working, try setting it to the specific device you're using.
If that doesn't work, check the volume mixer in your windows systray (the speaker icon in your taskbar next to the system clock).
 Make sure your headphones/speakers aren't muted in the mixer.
If that doesn't work, check if your headphones/speakers have a physical volume control or cutoff switch that needs to be adjusted.
If that doesn't work, QUIT DISCORD completely and then re-run it.  Sometimes it's just a Discord network issue.