This problem has occurred on my iPhone 5 with all versions of iOS that have been available for it.
When you want to do voice dictation you hit the microphone key on the keyboard, the microphone iPhone appears and you hear a beep, And you start talking in the icon flashes as it hears the sounds. When finished you hit the Done button and your transcribed speech is returned. This is the behavior when things work properly.
Frequently when I hit the microphone button, The microphone icon appears But there is no beep to let me know it has started listening. However the icon does flash in response to sound. When I'm done speaking and I hit the done button, no text is returned. The next time I hit the button it will work - every time.
Under continuous multiple use of hitting the microphone button the problem does not happen. It only happens after some period of nonuse of the feature. But as I said the second attempt after the failure always works.
Is anyone seeing this?