Center and surround speakers work, but front right and front left speakers do not put out any sound
I had the same problem. Set the reciever on 6 channel stereo. On this setting, you should be getting the same sound out of each speaker. Isolate which speakers aren't getting sound. Now, on your speaker switch (the A/B button), select A. If that doesn't fix it, select B. If that doesn't fix it, try A and B (if you can on your reciever).
Now, when you switch back to pro-logic, make sure your a/b channel switch is in the position that worked in 6 channel stereo. Yamaha recievers have a memory, and when you switch various modes, it makes auto adjustments based on what you had set in the past.
Hope this helps