ED,
It sounds like the volume of your internal speakers is too loud in relation to the mic volume. Try this:

Connect the loop outs on the PSR to the audio INs of an external mixer, then take the MIXER outs, and run into the Loop IN on the PSR. If the mixer has dual outputs, send one set to the main speakers/amps and the other will be the signal sent to the Yamaha internal speakers. In this loop fashion - the volume knob on the PSR is now a master volume for the MONITOR sound only (internal; spkrs) Neat, huh? You can actually turn the monitors OFF to do a sound check or avoid feedback .... works like a charm - wish the PA80 had it.