How to fix?
When you record each track, make sure that the level never goes up to zero on the meter. If it hits zero, it means that the recording is clipping. Clipping is when the computer can no longer accurately record the waveform. So instead of recording the shape of the wave, the upper and lower limits of it will be clipped in a straight line. So a tone sound like a sine wave would be clipped as shown below (the black line), instead of looking like the original wave (the grey line):
This results in a very distorted and unpleasant sound.
When you mix multiple tracks together, the combined sound is louder than a single track on its own. So you will need to make sure that the overall level of your project is also not clipping. If it clips, you will see the meters in Premiere go red like this when they are clipping:
You need to make sure there is no clipping anywhere in your signal chain, from recording to mixing the final audio.
Assuming the source material isn't distorted, you probably just need to reduce the level of each track in Premiere using the Audio Track Mixer (go to the Window Menu and select Audio Track mixer, and the name of the sequence you're working on).
The Track mixer adjusts the audio on each track. It is different from the clip mixer, which adjusts audio on each individual clip.
So you might have many clips on a single track, and instead of adjusting each one separately, you can just adjust the whole track once.