# Why is the video in beamer output pdf file playing without sound?

I have a video in my presentation with beamer class. The problem is that in the output pdf file the video is playing without sound. The code is as below:

\documentclass{beamer}
\usepackage[english]{babel}
\usepackage{multimedia}
\begin{document}
\begin{frame}
\movie[width=12cm,height=8cm,poster]{}{a.avi}
\end{frame}
\end{document}