Bug 1040215 - [Messages] Move initSentAudio out of enableSend r=azasypki...

...n
I believe that initializing audio inside ```simSelectedCallback``` won't work for "Resend message" case when user enters thread containing message failed to send and tries to resend it in this case "simSelectedCallback" won't be called. Don't you want to call it inside ```ThreadUI.beforeEnter\init``` for some reason?