It may be helpful slightly to amplify the passage Greg quoted in part earlier on.   The following translation of the relevant (Latin) passage from the Descriptio Cambriae of Giraldus Cambrensis was posted in the rec.music.early newsgroup back in 1998 by Margo Schulter.  In the first paragraph, Giraldus is writing of the inhabitants of the country we now call Wales.

"In their singing of music they do not sing uniformly [in unison?] as elsewhere, but in multiple manner, with many different voices and melodies. Thus in a crowd of singers, which is quite the custom for this nation, you will hear as many melodies as you see people,. and a distinct variety of parts, finally coming together under the soft sweetness of B-flat in one consonance and "organized melody" [likely organum,polyphony, apt harmony]

Also in the northern parts of Britain, that is, beyond the Humber and around York, the people who inhabit these parts use a similar kind of singing in "symphonic" harmony [i.e. based on the "symphoniae" or concords]: but with a variety of only two distinct melodies and parts, one murmuring below, the other equally soothing and charming the ear above. Yet in both nations this special style has been acquired not by studied art but by long usage, so that it has now become as it were a habit of second nature. And this has now become so strong in either nation, and taken such firm roots, that one never hears simple [unison?] singing, but either with many voices as in the former [Wales], or nevertheless at least two as in the latter [northern England]. And what is yet more marvellous: even children, and indeed infants, almost from when they first turn from tears to songs, follow the same fashion of singing.

Since the English do not generally use this manner of singing, but only the northerners, I believe that it is from the Danes and Norwegians, who often used to occupy these parts of the island and were wont to hold them for long periods of time, that the inhabitants have acquired likewise their affinities of speech and their special manner of singing ."

With reference to the sweetness of B-flat, Ms. Sculter added the following comment:

"One question raised by these passages relating to ensemble singing and harp music alike: just what does Giraldus mean by his references to the sweetness of B-flat? Young suggests that this could refer to the use of B-flat in an octave-species based on F or G, avoiding the tritone; at any rate, in the hexachord theory introduced by Guido d'Arezzo around 1130, the "soft" hexachord (as f-d') is distinguished by its use of Bb rather than B-natural (both notes being integral to Guido's system). Composed polyphonic pieces such as conductus from this same period around 1200 centering on F or G often feature the consistent use of Bb. Nevertheless, Hughes, ibid. pp. 316-317, wisely cautions that the meaning remains uncertain."

References:
Percy M. Young, A History of British Music (Ernest Benn, Ltd, 1967)
Dom Anselm Hughes, "Music in Fixed Rhythms," in Dom Anselm Hughes, ed., New Oxford History of Music, v. 2: Early Medieval Music Up to 1300 (Oxford University Press, 1954)