Not knowing what you plugged and where you plugged it - my best guess would be that you plugged a line output from the PC into a microphone input on your mixer desk.
This would be the equivalent of using a fire hose to fill a cup with water.
The line output signal is some hundreds of millivolts whereas the microphone input will be expecting a signal with a few tens of millivolts at most. The result will be that the PC is overdriving the microphone input and distortion will be heard.
The input on the mixer desk (signal level and impedance) needs to match the audio source that you have. The mixer desk may have a separate connector for line and microphone signal types, or a configuration switch/button (may be marked PAD). The front-end GAIN control may also need to be reduced.
The rule we have at our church is for the mixer desk to be configured as microphone inputs (low level balanced) and to use DI (Direct Injection) boxes to convert any line level signals into balanced microphone-type signals. This works great for things like laptops and ipod/MP3 players as everyone now knows to plug the device into the little black boxes on the platform and things work as intended!