All of Hildegard's 77 works (sequences, hymns and the Ordo Virtuum)
are monophonic.  The instrumentation, drones and other devices used
to flesh out the music you have heard on recordings are added by the
interpreters.  There is a variety of ways to approach this, and none can
truly be considered "right".  Hildegard's tunes may have been performed
insturmentally as well, but she didn't leave specifics as to how this was
achieved in her time.  For a reliable summary of techniques along these
lines and an excellent edition of some of her works I would refer you to
"Sequences and Hymns" edited by Christopher Page (Antico c.  1983).  The
primary sources for her work in it's original form would be "Schriften der
Heiligen Hildegard von Bingen" ed.  by Johannes Bulher for texts (and
visual art, Insel-Verlag, 1922) and "Lieder" ed.  Barth, Ritschler and
Schmid-Gorg (G.  Mueller, 1969) for music.  These will be of limited use to
you as the commentary is in German and in the latter case the notation is
give in mensural style, but they help to show you what we have to work with
in terms of her legacy.  You will also do well to investigate some of her
writings in order to understand her state of mind and place in the world-
for that I suggest "Hildegard of Bingen:  An Anthology" (SPCK, 1990).
Incedentally, to look her up in library databases nowadays you will find
the most entries under "Saint Hildegard"; the "von" or "of Bingen" is
usually omitted.  TMK there is no "e" at the end of her name.

I agree that the old Eulenberg and Kalmus scores are quite difficult to
read and are often uncertain as to how the rhythmic textures are supposed
to inter-relate.  That is, in part, due to the fact that they are prepared
from published sources which first appeared in the 1870s and 80s.  As good
as these were when they first appeared, we can do can better now and new
editions are certainly needed as the music has grown considerably in
popularity.  But good and new editions are not available in every instance.
For the Machaut there is both an edition and a valuable essay available
seperatly from Oxford University Press by Daniel Leech-Wilkinson:
"Machaut's Mass:  An Introduction" (OUP 1990) is the essay, and the edition
is titled after the work.  For a valuable second opinion I would also
explore Denis Stevens' controversial 1973 edtion of the Mass (also OUP)
which has the audacity to add instrumantal parts.  You may have read
Stevens' 1975 book "Musicology" (also OUP) which describes the hoops a
musicologist has to jump through to make an honest edition of an early
work.  At CCM the book was required for our courses; if you haven't read
it, I suggest it.  I'm afraid the news is not as good in the case of
Perotin.  The most accessible score of "Viderunt Omnes" may be found in
"Perotin:  Works" ed.  by Ethel Thurston (Kalmus, 1970).  While it is an
excellent edition and very legible transcription, the small size of the
publication itself makes it a challenging tome to lead from.  While I have
not yet seen it, the Musica Gallica division of L'Oiseau Lyre Press in
Paris is preparing a brand new multi-volume, multi-lingual edition of the
most important Notre Dame Organa manuscripts (notably Wolfenbuttel 677 and
1099) under the watchful eye of editors Edward Roesner, Thomas Payne and
others.  So far it runs to six volumes, and may (or may not) contain the
works you need.

One thing to bear in mind as you approach this music is that it is
impossible to 100 per cent reproduce the rhythmic flow of mensural notation
into the strict corset of standare Western notation.  Also, there are
signs in the manuscripts which are not always understood, and there was
ornamentaion used at the time which we do not have much information about.
For one man's opinion as to the latter, I would strongly recommend Marcel
Peres and Ensemble Organa's 1995 recording of the Machaut Mass (Harmonia
Mundi HMC 901590).  While the jury is still out on Peres' approach, I feel
it is,at least, a step in the right direction.  It might be useful for you
also to listen to recordings of harmonized chant, where the choir matches
the melody to a third or fifth above or below.  There is no clear cut
science to this, and every abbey has it's own approach to the practice, but
it goes back to even before the time of Organa, and may have been the
starting point of Organa itself.

Hope it helps,

Uncle Dave Lewis
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