Complex sentence: The great power of this fact is that any musical work can be played or sung in any key--it will be the same piece of music, as long as the intervals are kept the same, thus transposing the melody into the corresponding key.
Split into simpler sentences: The great power of this fact is that any musical work can be played or sung in any key. It is the same piece of music, as long as the intervals are the same--thus transposing the melody into the corresponding key.