sound effects mute after long games

Hi all.  I just had a question.  After I'm playing a longer game (1 and 1/2 hours or longer) the sound effects of the game stop so all I hear is the music.  This includes unit acknowledgements, "construction complete" messages and the like, and sound effects of the ships.  It's not a big deal, but I was just wondering why it did this.  Is this a bug?  Thanks.