Unfortunately this is not a specific answer... From the concepts below I have tried to illustrate interesting insights about the "multi colored" nature of this term and hence give a better understanding of what it may mean in different contexts...
Unfortunately, the word "sonority" (like many musical terms) developed a lot of artistic baggage as it was used to describe similar (but different) things during the evolution of western music. Conservatories are divided with strong partitions (ivory towers) between their departments. Each department has a different artistic vision. These visions sometimes play out together as pedagogic feuds, (that is a good thing ;-) but most of the time everyone keeps to their comfortable inner circles... (and so the musical definitions get fragmented even farther)
If your are looking for the definition of a term, you need to first think about context. What kind of music and style are you trying to describe?
For instance, the term "mode" means one thing to a jazz musician, but to a scholar of medieval and renaissance music it would mean something completely different! (painful personal experience taught me that lesson ;-)
Being a composer, my kind of mathematical base definition of "sonority" would be:
"The psycho-acoustic perception of a collection of 2 or more individual tones (frequencies or pitches) sounding together, with all pitches having only simple harmonic relationships (ratios) with each other, the result of which mimics the sound of a single fundamental pitch sounding alone"
The "perceived tone" in my above definition would actually be the root fundamental harmonic of the entire collection of pitches sounding together... Instead of hearing a chord, if it is simple enough (i.e., sonorous) your mind wants to simplify it to a single "root" note (with a pleasant tonal color).
This is a side track but btw, there is no such thing as one single note... Our brains trick us... All musical instruments produce notes (tones) that contain harmonics (pure notes that sound all by themselves). Even a really precision HP sine wave signal generator produces very faint stray harmonics (called distortion in that case.. ha ha.. engineers call mother nature bad names). Once released into an acoustic space, that same single HP generated sine tone hits things as it travels to your ears and breaks up into other sine waves producing more "tones" (just as ripples in a pool of water can break up into multiple overlapping waves).
Our brains trick us into "hearing" sound as one note (with a characteristic tone). A flute playing A440 sounds much different than a clarinet playing the same note... Piano tuners learn how to selectively listen to these individual harmonics as separate tones. Only a few of us were born with that ability though. Most of us hear sound as a collective "sonority" oops! another use of that elusive term... ;-)
The above two paragraphs are a related topic (in reverse) but OK... back to main topic now...
My definition of the word sonority above is my own based heavily on my artistic style(s), but if you are a blues musician you would have a completely different view! For blues the I chord, is a dominant 7th chord. In fact the entire I-IV-V-I blues progression is all dominant 7th chords!
So to a blues musician a sonorous chord could be G7 (in the key of G major)! Yes! The 3rd & flat 7th chord members form a tritone interval which many a theorist would say is not sonorous at all but the most dissonant of all possible combinations! (to each his own) Country blues singers sing that stuff in microtones which stretches our understanding of what sonority means even farther out!
Now, do you see the problem about this and many other musical terms? I think this confusion is beautiful... That is how new art is discovered!
Artistic concepts are heavily influenced by changing human taste or fashion... The nature of music is mathematical however so someone knowledgeable in psychology/cognition and music theory might try to nail down a general definition (like I did above for me). Unfortunately that won't change common usage of the word in all those different cultural settings... Only collective life experiences can do that... Such is life...
I guess my answer does not help much.. eh? lol I hope it makes this discussion more interesting and helps to expand our musical consciousness!