THE MOST INTERESTING recent development in early music has been a willingness on the part of many performers to listen carefully to the secrets their instruments are trying to tell them.

The postwar generation of early music performers, handicapped by unsuitable transcriptions, modern conservatory training, and the pressure of proving that early music could compete in the current concert scene, had difficulty transmitting an accurate feeling for pre-Baroque music. And their instruments were often just generalizations of historial models.

But there's been a renaissance in Renaissance music. It revolves around a new breed of craftmen and the instruments they produce - precise copies of extant examples or intelligent speculations on surviving iconography. These reproductions have a lot to say to performers. To the extent of their fidelity to the originals, they become a liberating rather than a limiting influence on performance practice.

Over 100 antique instruments, orginals and facsimiles, may be heard on a fascinating two-record set on the Angel label - Instruments of the Middle Ages and Renaissance (SBZ-3810) - conceived and produced in England by the late David Munrow, who, with his Early Music Consort, was a vigorous and talented proselytizer for early music until his untimely death last year. The records canvass about 400 years of early instrumental practice, introducing listeners on an item-by-item basic to the pleasing and sometimes exotic sounds of the gemshorn, psaltery, rebec, orpharion, rackett, rauschpfeife, serpent, and tromba marine, among many others. Packaged with the records is the most elaborate set of liner notes yet attempted in the field - a 96-page large-format illustrated guide that also stands on its own merits as a separate publication of the Oxford University Press (paperback, $12.95).

Munrow emphasizes performance aspects - how are these curious instruments actually used, what kinds of music do they play, how are they held and tuned, and what kind of technique is required to play them? Many of the examples have also been cross-referenced with other records produced by the Early Music Consort, and this helps offset an obvious problem with an all-instrumental approach: Although the instuments are demonstrated on suitable music. Munrow often must leave part of their potential to the imagination. For example, fanfare music for two 15th-century business is taken from the lower two parts of Guillaume Dufay's Gloria and modum tubae. Interesting as a fanfare, but we must turn to Munrow's Dufay recording, Seraphim 60267, to realize how these straight trumpets sounded while supporting the top vocal lines. An all-instrumental early music recording avoids the irresolvable problem of vocal style in an irretrievable era. But since the human voice was then considered the most nearly perfect instrument, and the repertoire reflected this bias, any purely instrumental demonstration implies a whitish lie larger than the truth it can impart.

On these records, acceptable anachronisms occur in the earliest periods, where Munrow chooses surviving folk instruments as substitutes, but it is surprising to hear an 18th-century violin used in 16th-century music, and a number of undistinguished production-line woodwinds. (And to play the music of William Byrd on a viol built in 1704 is an expedient one would expect only under emergency conditions.) Indeed, many of the instruments Munrow uses are unfussy, modern-pitch versions rather than the replicas we might have hoped to hear.

If not a Rosetta stone, Instruments of the Middle Ages and Renaissance is certainly a revelation. But those not interested in a musical checklist - even one as richly produced as this set - might try some of the other records of the Early Music Consort - for example, The Art of Courtly Love (Seraphim SIC-6092), a spectacular tour through the secular music of the 14th and 15th centuries, including some shocking examples from the 14th-century avant-garde. Or they might acquaint themselves with another vital English ensemble, Musica Reservata on Philips - or the Studio der Fruhen Musik, whose Telefunken and Reflexe releases have achieved an exquisitely sensitive flamboyance, especially in their latest investigations into the music of Bernart de Ventadorn, Francesco Landini, Oswald von Wolkenstein, and Guillaume de Machaut. In America, the Waverly Consort has recently given us two smoothly produced recordings, Dounce Dame (Vanguard 71179) and Las Cantigas de Santa Maria (Vanguard 71175) that show off its professionalism and dramatic flair.

As an alternative to Munrow's accompanying booklet, one might wish to explore The World of Medieval & Renaissance Musical Instruments, (Overlook Press, $16.95) a colorfully illustrated 136-page volume published recently. The author, Jeremy Montagu, has been active in reviving the art of early percussion (his research has inspired the invigorating percussion on the Munrow recordings) and his book has the advantage of being drawn entirely from comtemporaneous sources and iconography.