One way in which nuns could have performed music which lay out of their vocal ranges was of course through the use of instruments, and it is unquestionable that these forbidden entities were employed in the convents, despite all the regulations banning them; indeed nuns themselves wrote pieces using instruments. Most church authorities allowed an organ, a harpsichord for study purposes, and a viola da gamba to play the bass line. More surprisingly, many nuns chose to play the bass lines on the trombone, with or without the consent of their superiors.
But the lower parts were not always performed instrumentally. It is no secret that some women can sing quite low, and given the circumstances in the convents, this kind of talent would have undoubtedly been cultivated. Proof of such women has come down to us with the names of female "basses"; listed among the nuns. Thus, if we accept the possibility of a woman being able to sing down to a low C (a low but not impossible note for a female voice), the performable repertoire becomes much larger. Certainly tenor parts rarely go below this, so anything written for canto, alto and tenor would fit within the range of women’s voices.
It is by now well established that transposition was a common practice in the 16th and 17th centuries. In particular, certain combinations of high clefs (the so-called "chiavette") indicated specific transpositions, usually downward by a fourth or a fifth. But in most cases, if such pieces are sung as written at the high pitch, they fit entirely, or almost entirely, within the range of female voices. On the other hand, when normal clefs are used, the soprano often does not go above d'' or e'', so that a transposition upward of a 3rd or more would enable women to sing all the parts. And it is interesting to note that two important treatises with instructions on how to transpose by unusual intervals, Cima’s Partito de’ Ricercari of 1606, and Lorenzo Penna’s Primi albori musicali of 1672, are both dedicated to nuns. It is probably safe to conclude that the nuns transposed their music to the pitch where they were most comfortable, often using non-standard transpositions, or no transposition where the clefs would normally indicate it.
Finally, the practice, surprising as it may seem to our ears, of singing bass parts up an octave (though preferably doubled by an instrument at the lower octave if chordal inversions would otherwise occur), was also demonstrably in use by nuns. Romano Micheli (c. 1575-1659), among others, documents this practice in his two collections of psalms for 3 voices (1610 and 1615), which give various suggestions for performance, including the possibility, when "used by the reverend nuns", of an ensemble of 2 sopranos and an alto "singing the bass an octave higher".