To what extent do English words sound like what they describe?

This is a very difficult (if not impossible) question to answer, since testing whether a word "sounds" like a concept is largely subjective. It is further complicated by the fact that humans will tend to "chunk" the memory of an auditory stimulus of a word with its associated visual stimuli. In other words, when one hears the word "book", one is very likely to simultaneously, automatically, recall the memory of the sight of a book. Therefore, one might be biased to think a word is onomatopoeic simply because one subconsciously already has an association between the sound of the word and its associated concept.

The easiest way I can think of to non-subjectively test the theory you propose is to take a group of non-English speakers (preferably those who don't even speak an Indo-European language), present them with English words and pictures of the concepts the words represent, and then ask them to match the English words with their associated concepts. I am not aware of any such experiment that has actually been performed, however.

This is further complicated if we accept that language evolution exists on a continuum: The current meaning of a word in modern English can have little or nothing to do with its earlier meanings. For example, about 1000 years ago, the word "book" (bóc) had a meaning more similar to "writing tablet" or "single sheet of paper", the latter of which doesn't have a "hard quality" at all.

Włodzimierz Sobkowiak did some research on this in the 1990s, studying English onomatopoeia's phonostatistics (the paper is available here). He took the words in Kloe's dictionary on onomotopoeia and statistically compared their phonetics to the rest of the English lexicon. Sobkowiak concluded that the words in the dictionary (that were specifically identified as being onomatopoetic) had statistically significanty different phonetic patterns than the rest of English, suggesting that not all English words are onomatopoetic.

Once you exclude onomatopoeia, the theory makes little sense.

Do the sounds things make influence the words that are used for them? Clearly this happens to some extent, as the cuckoo demonstrates, but this is onomatopoeia and we must discard it from discussion. Adjectives that sound like the sounds they describe? Again, we must discard them.

With what remains, you are asking if the tonal qualities of the words have a tendency to match the nature of the thing the word stands for. That relies heavily on the meaning that you attach to tonal qualities, and I would suggest that those meanings are likely to have formed the other way around; for example, lethargic has a heavy, slow feeling to it because of its meaning, instead of meaning something slothful because of the way it "feels".

Synonyms are another good way of testing the theory past breaking point. "Amanuensis" has a very rounded, flowing sound to it; "Secretary" is full of stops and sharp edges. The emotional quality of the tones are very different, but the words mean the same.

In short, I don't think this theory holds much water.

Contrived examples are contrived, and not exceptionally carefully thought through at that.

1. Water literally has a stop right in the middle of it. War flows much better.
2. If book has a "sort of hard quality to it", then so does e-book. Oh, and boob.
3. Think of all the minimal pairs. Thought, sought, caught, taught, bought, brought don't make you think of pretty much the exact same thing, do they?
4. Since you expressly mentioned all languages, let's have a quick look at the word for water in just a few of them:
   • 'wɔːtə
   • ˈʋaːtər
   • ˈvasɐ
   • vɐˈda
   • vɔˈda
   • ˈakːwa
   • o

Does eau flow better than aqua? Yes? No? Why would they flow differently to begin with? It's still the same H₂O.

Onomatopoeias are onomatopoeic; non-onomatopoeias are not. If all words were onomatopoeias, there would be no need for the term onomatopoeia to begin with. We'd just refer to onomatopoeias as "words".

This is an ancient question with much attendant scholarship rejoicing in the name of 'phonosemantics', qv http://en.wikipedia.org/wiki/Sound_symbolism

Friendly introdution with bouba and kiki here: http://www.visiblemantra.org/phonosemantics.html