https://msdn.microsoft.com/zh-cn/library/ms720149.aspx
By default, a voice is set to SPVPRI_NORMAL which means that it will wait until other voices in the system have completed before it will begin rendering its input queue.
A voice set to SPVPRI_ALERT will interrupt a normal priority voice by stopping the normal voice, rendering its own queue, and then restarting the normal priority voice.
An SpVoice with a priority of SPVPRI_OVER will simply render its data immediately even if another voice is currently speaking (they would both speak at the same time).
Applications can control the priority of a voice by calling ISpVoice::SetPriority.
ISpVoice::SetPriority参数：
SPVPRI_NORMAL:整台电脑按队列顺序播放
SPVPRI_ALERT:打断正在播放的SPVPRI_NORMAL声音后播放，播放结束后重新开始播放SPVPRI_NORMAL声音
SPVPRI_OVER:不打断其它声音，自己立刻播放，相当于同时（重叠、混音）播放