We give Puberphonia Eradication advisor training.
1.Audio speech training.
That feedback sound is ringing at a specific frequency. While ringing out the speaker(s) through its global equalization (EQ) settings, identify the frequency (i.e. 200 Hertz, etc.) and lower the intensity about 5-10 decibels at a time. That should begin to lower the ringing noise. You would be reducing the amount of sound throw of the speaker in that frequency range. When the throw of audio is heard again through the pickup of the microphone, you have created a feedback loop. The result is the feedback sound.