i think your samples have a too high volume.
decrease the volume in a wave editor before importing in Polyphone.
post a .sf2 containing only one of the problem samples, so we can see what is wrong.
regards bottrop
All samples should always be "normalised" to approx 1db below their peak value (use a .wav file editor to do this) before importing them into polyphone. If the samples are clipped and distorted before you import them then you cannot use them to make a good sound font.
In order to set the output volume each instrument and preset has an attenuator built in - use these to adjust their sounds to your preference. Note that presets which contain more than one instrument can still clip unless these attenuators are used properly.
If you still have problems then (as bottrop said) attach a sample which demonstrates the problem to your next post so we can see where you are going wrong.
My guess is that the user placed the samples in the instrument part but did not set a key range for each.
I think that might be the explanation for loud and distorted sound.