I'd guess your computer is set to 5.1 speaker output but has no center speaker. There seems to have been a change the default sound system a while ago that enabled multi-speaker sound output even if you don't have hardware acceleration turned on. However, the downside is that if your Windows settings don't actually match your speaker configuration, you may lose audio channels - in this case, you're presumably losing the front center channel, where speech normally is played.
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