The speak icon has disappeared from my text messages on Iphone 4s. How can I get it back?

Hey Mina and kousha,

Welcome to Apple Support Communities! Sounds like you want to use Dictation on your iPhone. Dictation is turned on as part of Siri, under Settings > General > Siri, so I'd check there and make sure it is turned on. You can learn more about Siri here.

iOS: Using Dictation

http://support.apple.com/kb/ht5176

Regards,

David