Limit audio buffer to 700ms.  This prevents the frontend locking up when trying to stop playing an audio-only stream (e.g. DVB radio).  Without this patch the buffer tends to fill to around the 1500ms, which is below the current 2000ms limit, and AvFormatDecoder never leaves GetFrame.

Normally AvFormatDecoder::GetFrame is exited when a video frame is decoded.  In this case the audio buffer is usually no more than about 500ms full.

Also, buffering less audio data means less lag is noticeable when altering the volume if software mixing is used.