wrong guitar clef
MuseScore uses the treble clef 8vb in the guitar staves, but guitar is traditionally treated as a transposing instrument. Most of guitar scores uses the simple treble clef and the notes sound one octave below the written notes.
There is an article in Grooves Dictionary about transposing instruments that confirms this.
As a workaround, when editing my scores I use to replace the original code for guitar in instruments.xml file with this:
In which I change the clef and insert a transposing code.