It is very difficult to diagnose this without knowing more details, but I will toss out some speculation. The
system may have two ports, one intended to accept
line level from a
mixer, and another for connecting a
microphone. The mic port would be much more sensitive with a whole bunch more electrical
gain.
If you inadvertently connected the
line level output of a
mixer input the
system's mic input, and if a mic was turned on at the
mixer, that could easily cause high amounts of
feedback (in your words, "high pitches"), and
distortion. The controls on the
mixer would be on a hair trigger.
If the
feedback was loud enough and lasted long enough, it is very possible to damage
speaker components. Most of the time, it is the high frequency
driver that gets cooked. That would explain the poor sound quality after you managed to get things working. You probably need to have a service technician come to check out the
system and make repairs.
This is why it is extremely import to make connections between mixers and amplifiers with the
system powered off. Make connections of microphones only with
mixer channels all the way down and muted. Always bring up
microphone levels very slowly initially, while carefully listening for the beginnings of
feedback.
Again, my comments are based on a lot of speculation, but also on a lot of experience.
