Hello wemfan,
Did you do Step#5 on my procedures?
Under the Setup Menu, go into Audio Processing and then into Speaker Configuration- You’ll then see a diagram on your tv of a 5.1 system with 5 speakers, a sub and a guy sitting on a couch. Then follow this procedure:
1. Use the left or right arrow key on your remote until the "Test Tone" area is highlighted (located in back of the couch) and hit enter to turn the test tone on (there’s a speaker symbol at the bottom left of the screen that will indicate if the test tone is on or off).
2. Once the tone is on, use the left or right arrow key to scroll to the FR/Front Right speaker and press enter. You’ll hear some pink noise being played through your FR speaker and you then need to set the following for this speaker:
Size: Set this to "Large" if you want this speaker to run full-range and "Small" if you’re using a sub and want to limit the low frequency output of this speaker. You don’t have a sub yet so set this to "Large".
Distance: Set this to the number of feet away from the speaker your listening position is located.
Trim: Set this to "0dB" initially. Later, you can go back into this section and adjust the relative volume/trim of all 3 of your speakers so they're level matched or set to your preference.
3. Repeat Step#2 for your C/center channel and FL/Front Left speakers.
4. Sit at your listening position and scroll through all 3 speakers and, using the test tone, adjust the Trim on them by ear until all 3 sound the best to you.
It sounds to me like your center speaker may have been set to "Off" by default. It may help you to read the Oppo manual on this subject starting on page 63. Please let me know if this fixes your issue.
Thanks,
Tim