Distorted sounds

Especially when using voices, I get a high scratchyness, which sounds as if it should be a speaker connection problem or a loose diaphragm on the speakers. However, my Bose speakers are fine with other music and other instruments in Musescore. I've been able to reduce the effect a little using an equaliser. I've also tried various combinations of volume on the computer, on MuseScore and on my speaker, to no avail. The sound seems to be integral to the .sfz file or MuseScore itself since it is also reproduced when exporting to mp3. Has anyone else come across this and found a solution please?