It wasn't Guido, but I would love to know what vocal colour and technique were cultivated in his day. We might consider anything before belle canto to be somewhat if not totally strange. (Think Marcel Perez.)
Perhaps a higher likelihood of a reedier, more nasal-forward sound, as that would make it more likely to be heard and understood, as with recited declamation and cantillation (for a more recent example, consider Abraham Lincoln as an exemplar of his era of public speaking: he reportedly had a penetrating reedy tenor public speaking voice, not the deep bass we've come to associate with him because of cinematic license). Aural sweetness as conceived in modern times was not necessarily as important a value as penetrating open air and large interior acoustics.
DDL's register is informed by the historical reality, though still delivered somewhat sweetly with microphones in mind rather than actually projecting to be heard by a large crowd in, say, the Cooper Union or at the Gettysburg cemetery.
By contrast, Walter Huston's bass rendition in 1930 was a powerful influence on popular culture, reinforced by Raymond Massey's bass in 1939. Sam Waterston's baritone for Ken Burns' "The Civil War" was still quite mellow and intimate; it was far from a public speaking voice.
The first real voice treatise is considered to be Tosi's (1652-1732) Opinioni de’cantori anthichi, e moderni, so fairly late in the game. Zacconi (1555-1627) and Caccini (1551-1618) wrote general treatises that the deal with the voice in some detail.
For chant, an important source for singing style is Conrad von Zabern's De modo bene cantanti (1473) Translation
@Jeffrey Quick: Grazie! Conrad von Zabern's De modo bene cantanti is exactly what I was looking for; it seemed more geared to singers of Gregorian chant than the other works you mention.
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