Now Android's "OK Google" offers send message from voice control. It is supportable for Whatsapp, Viber, WeChat, Telegram and next plus. 

Previously Google's Voice feature allow send message via email, text message and hangout message. Now google also provide voice control for send message via voice.

NOTE: Install latest Whatsapp/Viber/WeChat/Telegram or Next Plus application and Google Application




Play Store Links for Above Application:

1. Whatspp - https://play.google.com/store/apps/details?id=com.whatsapp
2. Viber - https://play.google.com/store/apps/details?id=com.viber.voip
3. WeChat - https://play.google.com/store/apps/details?id=com.tencent.mm
4. Telegram -  https://play.google.com/store/apps/details?id=org.telegram.messenger
5. Next Plus - https://play.google.com/store/apps/details?id=me.nextplus.smsfreetext.phonecalls
6. Google - https://play.google.com/store/apps/details?id=com.google.android.googlequicksearchbox

Below comment is used to send message via Whatsapp.


STEPS:

Step 1:  Open Google application on android mobile
Step 2:  Say "OK Google" or click "mic" icon at right top in the application
Step 3:  Ssay "Send Whatsapp Message" or "Send Whatsapp Message to 'Whatsapp Contact                             Name(Ex: Sivakumar, Sujith Krishna') " .
Step 4:  Type your message in the text area or Say your message as voice..
Step 5:  Say "Send Message" or click send button.
Step 6:  Now the message will be send and you will be notified.


SCREESHOTS: