Beamer使用media9宏包插入音视频时，我想让音视频在这一张frame的某一个slide自动播放，于是写代码如下：

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\begin{frame}
	\onslide<1-2>
	This is the first line.
	
	\onslide<2>
	This is the second line with audio.
	
	\onslide<2>
	\includemedia[
	addresource=audio.mp3,
	activate=pageopen,
	flashvars={
		source=audio.mp3
		&autoPlay=true},
	transparent]{}{APlayer.swf}
\end{frame}

这样并不能实现预期的效果，正确写法如下：

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\begin{frame}
	\onslide<1-2>
	This is the first line.
	
	\onslide<2>
	This is the second line with audio.
	
	\alt<2>{
		\includemedia[
		addresource=audio.mp3,
		activate=pageopen,
		flashvars={
			source=audio.mp3
			&autoPlay=true},
		transparent]{}{APlayer.swf}
	}{}
\end{frame}

背后原因未知。