Problem solved. It wasn't exactly what you advised, but I did find the solution in the section of the Yamaha manual that you directed me to, so thanks.
The solution was to set the receiver's sound field program to 2 channel stereo (page 34 of the manual). It had been set to Straight, which is the setting that is automatically selected for PCM sources above 48 KHz.
A couple other side notes:
- On the TV, when I select External Speakers, the only output format that it gives me is PCM. Dolby Digital is greyed out and can't be selected. Not sure why. For most programming, I see a PCM indicator on the receiver and the audio is fine. For some programming though, I see a Dolby Digital indicator, and that's when I had the problem.
- On the receiver, I had already set it to 2 Speaker mode (as suggested by gajCA), so that alone wasn't enough to solve the problem.
Thanks for the responses and for steering me in the right direction.