The one at 0:31 is the most harsh, IMO.
That must be the surprise factor, it is the first entry of this voice. Nothing harsh about it, though it may sound strange if you are not familiar with organ music.
And what the heck is a Sexquialter register for the cantus firmus? (You don't have to answer that- I just think it's fun to say)
In organ works, the cantus firmus is most often the undecorated chorale melody that floats under, or above, or even in the middle, of the other voices. We organists (hehehe
) like to choose a register with a nasal quality for this, to make it really stand out. The unique sound is produced by a pipe that does not have a length of 2, 4, 8, or 16 feet, like all others, but somewhere in between. The sexquialter is a typical Baroque register with a length of either 2-2/3' or 1-3/5'. I use that a lot on 'my' organ.