Thank you all for your suggestions.
Protagonist - Hmmm...good thought. Not sure if the DVd player has the karaoke option, but it' something I can check out.
overlander63 - the fade and balance is set to center, I did make sure of that. I don't remember seeing the "vocal" option though. Not sure where to look for this. I've got the pdf version of the Sony media player User Guide, and the only mention of the word "vocal" is when you set the "equalizer curve". At this point, I'm totally out of my league with this.
We never had this problem before. It's frustrating. If something is broken, it's under warranty - so I'm not worried about that. If necessary, I guess it's just time to call the factory on this.
Thanks again for your help.