Right click on the speaker icon (usually at the bottom right of your monitor screen)
Click on 'video and audio' on the left side of the settings window.
Use the 'microphone' drop down menu to select your recording device. This is usually the 'default' recording device that you had set previously in the sound control panel.
Now try joining some rooms. You should be able to speak to people in the room and should be heard clearly.
If others say your audio is too low or too loud, you can adjust it within Windows recording device properties.
Go back to the recording device control panel and click on the 'properties' button at the lower right.
Now select the 'levels' tab on the properties window. Slide the volume slider to the right to increase the volume of your microphone. Slide it to the left to decrease the level.
If you are told that your 'mic is too low' even though the microphone slider is the whole way to the right, use the 'microphone boost' slider to exponentially increase the level.
Note: the 'microphone boost' level may 'self adjust' as you speak to prevent feedback if you are in a 'multi-channel' room.
To prevent this from happening you should wear headphones so your microphone does not 'pick up' your speakers while you are talking. As above, this should only happen (if at all) if you are in a multi-channel (3 mic) room.