Did John Calvin discourage harmonies in congregational singing? In many traditional Reformed churches, the songs found in hymnbooks and psalters (collections of Psalms set to music) are written with four parts: a melody/soprano line on the top, and alto, tenor, and bass harmony lines below.  For example:


  


I was thus surprised to learn, while listening to a lecture by Kevin Twit, that John Calvin opposed the use of harmony in congregational singing:


  Calvin, for instance, did not think it appropriate to sing the Genevan psalms with harmony, because that was a violation of the unity of the body.
  ["Does Musical Style Matter?" MTW Global Missions Conference 2013]


In which of his writings did Calvin express this preference?  Was "the unity of the body" the only basis for it?
Did John Calvin discourage harmonies in congregational singing?

He did not discourage congressional singing, but it seems that John Calvin did discourage congressional singing in harmony.


  In the area of church music, Zwingli, although an amateur musician himself, had not set down any of his ideas of congregational singing. Calvin, however, was well aware of the power of music and soon stated his ideas "in. Keeping with the austere simplicity of his religious views."(Re p.J58) He referred to St. Paul as his authority in advocating the singing as well as the reciting
  of the psalms, and he permitted no other texts to be used for this purpose. In keeping with his views that the psalms should be understood by the congregation, he adopted the use of French translation, in verse. He found no objection to singing these and other psalm adaptations
  to German melodies borrowred from the Lutherans. His main stipulation in regard to a melody was that it should be the equal of the text in majesty. (Re p.J59)
  
  Luther demanded of church song that it should be predominantly (but not exclusively) congregational, that it be in the vernacular, and derived from scripture, so that through the medium of song, the Word of God might remain among the people. Unlike Calvin, Luther had no objection either to the traditional Catholic chant or to contemporary part-music. Calvinists on the other hand, steadfastly refused to countenance any of the trappings and ceremonies of Romanism.(Sc xvii) They, too, insisted on the uses of the vernacular and, with the same motive they avoided such musical elaborations as tended to make difficult the understanding of the words being sung or to distract attention from these by linking them with polyphonic settings which openly displayed vocal skill, appreciation of which they regarded as
  a species of sensual enjoyment.
  
  Calvin states his idea of the role of singing and of the danger of distraction in Book III of the Institutio, (chapter 20, sections 31 and 32): 
  
  "It is fully evident that unless voice and song, if
  interposed in prayer, spring from deep feeling of heart,
  neither h9.s any value or profit in the least with G:od. But
  they arouse his wrath against us if they come only from the
  tip of the lips and from the throat, seeing that this is
  to abuse his most holy name and to hold his majesty in derision.
  
  Yet we do not here condemn speaking and singing but
  rather strongly commend them, provided they are associated with the heart's affection. For thus do they exercise the
  mind in thinking of God and keep it attentive , unstable
  and variable as it is, readily relaxed and diverted in different
  directions, unless it be supported by various helps.
  Moreover, since the glory of God ought, in a measure, to
  shine in the several parts of our bodies, it is especially
  fitting that the tongue has been assigned and destined for
  this task, both through singing and through speaking, For
  it was peculiarly to tell and proclaim the praise of God.
  But the chief use of the tongue is in public prayers, which
  are offered in the assembly of believers, by which it comes
  about that with one common voice, and as it were, with the
  same mouth, we all glorify God together, worshipping him
  with one spirit and the same faith. And we do this openly,
  that all men mutually, each one from his brother, may receive
  the confession of faith and be invited and prompted by his
  example.
  
  And surely, if the singing be tempered to that gravity which is fitting in the sight of God and the angels, it both lends dignity and grace to sacred actions and has the greatest value in kindling our hearts to a true zeal and eagerness to pray. Yet we should be very careful that our ears be not attentive to the melody than our minds to the spiritual meaning of the words.
  
  Such songs as have been composed only for sweetness and delight of the ear are unbecoming to the majesty of the church and cannot but displease God in the highest degree." (Ca pp.894 and 895) - John Calvin and music: the rise of protestant psalters in the sixteenth century


Tim Montgomery goes on to say:


  Bourgeois favored part-singing. Calvin did not. His opposition not only ultimately dismayed Bourgeois so much that he left Geneva in 1557, but the use of unaccompanied melodies characterized the Reformed singing
  of the Psalms, in most places, for centuries. (Ge p.?4)
  
  In spite of Calvin's opposition to harmonization, four part settings were made by Bourgeois in 1547. His latest known publication (1561) included settings a 4, 5, and 6, described as suitable for instruments as well as voices, of eighty-three of the melodies he had arranged at Geneva. While none of his harmonic versions found permanent acceptance, his melodic settings influenced the harmonization of other composers, especially Goudimel.


Ca, pp. 894-895: Calvin, John. "Institutes of The Christian Religion."
translated by Ford Lewis Battles. The Library of Christian
Classics ed. by John T. McNeill. Vol.'s XXI and
XXII, (Philgdelnhia, The Westminster Press,Vol. XXI,1960,
Vol.XXII, 1954.)