If you are deaf, why can you hear me talk?
Being deaf is a spectrum, and not everyone who is deaf has no hearing. I wear hearing aids which connect to my computer, and this allows me to hear you and the game. Some deaf CS players have no sound or may only catch bits and pieces. Those with hearing aids may lose directional sound. This means they can hear an enemy walking for example but cannot tell if the enemy is to the left or the right.
If you are deaf, how can you talk?
It is a common misconception that all deaf people are not able to speak. The truth is, many of us can! Some may speak with a deaf accent, too loudly or too softly. For those who lost their hearing late or went to speech therapy, their speaking may be completely indistinguishable from someone who has all of their hearing.
Should people yell so you can hear us better?
If you are interacting with someone who is deaf DO NOT YELL AT THEM. This does not help. The most important thing when communicating with someone who is deaf is word recognition, not whether they know you are speaking. Word recognition is the number of spoken words that are understood by the listener. Speaking clearly and calmly and making sure to pronounce each syllable is important. Using short phrases or simple to understand words helps too.
Make sure to make your calls as concise as possible.

Not all deaf people can speak, not all deaf people have hearing, not all deaf people have access to hearing aids. Before you flame someone for not comming, try typing to them :)