If Gregorian chant had truly had only nuances of duration, then it had to assume a completely isolated position. Something like this was and is namely unknown to the vocal monophonies of other Christian churches: Byzantine, Greek, Syriac, Coptic, etc. (Jan van Biezen, "The Rhythm of Gregorian Chant")
Cardine argued that long and short notes are not in a strict proportion of time values with respect to each other, but rather degrees of nuance bound to the natural speech rhythm of the syllables. This opinion, which is still fundamentally free-rhythm, has since been adopted by many scholars. Jan van Biezen has proposed an alternative view that harmonizes both the testimonies of medieval musicians and the paleographical shapes of the neumes with a historical perspective enlightened by experience of church music through time in both the West and the East. The resulting rendition of the chant is essentially metrical. Chief among the discoveries is that of an underlying tactus, or beat, in the chant. This feature naturally brings Gregorian chant into historical and musical consistency with chant systems of other ancient churches such as the Greek and the Coptic. (Kevin M. Rooney, translator's preface)
I would never venture a guess on how any chant was sung in the fifth century. We have no notated sources from that period, and I have repeatedly emphasized the importance of following the oldest extant sources. That is as close to the authentic traditional rhythm as we can get without more conjecture than I would be comfortable with. But they get us to something singable, and that's the important thing! I have no interest in creating something that never existed. We've had quite enough of that already with at least half a dozen different antimensuralist approaches from Solesmes and elsewhere in just over a century, and now apparently cycling back through them. I'm content with the oldest sources, although I will occasionally deviate from L, especially if Chartres and all of the SG MSS from the 9th–11th centuries give a different reading. Scribes make mistakes, and the scribe of Laon was no more infallible than any other.
I believe, using the Laon neumes, a schola would be able to chant with a consistent rhythm using the most up-to-date mensuralist interpretation of this manuscript family.
For a good book on Greek musical theory and Boethius's De Musica's (and other's) influence on mediaeval European music and its theory look into getting a copy of The Critical Nexus by Charles Atkinson (Oxford, 2009). Atkinson is Professor Emeritus of Musicology at Ohio State University and has been (2007-2008) President of the American Musicological Society. A very eye-opening book about our ritual chant's origins....Boethius's treatise...
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