"No more sound" or "no sounder"? Or are both valid?

Question

I'm writing an essay for an English subject about the film The Shawshank Redemption.

I've used the sentence

But though the Warden presides over many prisoners, he is morally no sounder and stands less chance of reform.

My teacher has corrected no sounder to no more sound, but Microsoft Word identifies the latter phrase as grammatically faulty. No more sound does sound odd when isolated, but I can't put my finger on why specifically.

There are mixed opinions online, to the point that citing them here would do little good.

What are your thoughts?

Answer 1

The evidence below strongly suggests that sounder is a valid comparative form of the adjective, sound.

• “Unhappy being,” said the Abbot, “who hast no better subject of pleasantry than that which should make thee tremble — no sounder jest than thine own sins, and no better objects for laughter than those who can absolve thee from the guilt of ...
  Sir Walter Scott, The Abbot, 1820

• We are willing that religion should be judged by its fruits ; there is no sounder principle than that ; but be sure to judge it by its fruits; do not judge it by anything else.
  The Nazarene, 1843

• This argument is obviously no sounder than the following: ‘How can language describe the world?’—‘Because the world is language-like—it is linguistic’, and no sounder than: 'How can the English language describe the world?—‘Because the world is intrinsically British.’
  Karl Popper, Conjectures and Refutations: The Growth of Scientific Knowledge, 1963

• There is no sounder investment than property. Land. You all know that a dollar won't buy what it used to buy. A dollar might not be worth but fifty cents a year from now.
  Cormac McCarthy, Child of God, 1973

And if we examine the noun phrase, sounder morals, Google Books yields over 240 instances of this usage.

• but it is refreshing to see people young and old alike can have a hero in this day and age with a sound work ethic and even sounder morals.

• Lincoln was not only more available than Seward, but he had sounder morals and better judgment than Seward, or any of the others. He was far better equipped ...

• The English romances have traditionally been said to embody sounder morals' because they drastically changed French concepts of love, sex and marriage and were 'less lascivious'.

• ... under our Universities imparting Higher Education and training leaders, teachers, and professionals who would dedicate their services in the cause of better education, sounder morals, nobler manners, sweeter life and happier surroundings ...