Jeffreys's own compositions are greatly outnumbered by his copies of music by other composers, some English (Walter Porter, John Wilson), the majority Italian, which he made as part of his duties for Hatton. The Italian music survives in two manuscripts in the British Library and four in the Bodleian Library, Oxford. The British Library manuscripts are the most extensive, one (Add.31479) containing 110 motets with basso continuo for one, two and three voices, and the other (Mad.Soc.G.55–9) containing a companion set of 49 motets for four and five voices. The composer is rarely named, but virtually all the pieces can be identified (Wainwright, 1997). Most of the music appears to have been copied from printed books purchased by Hatton in 1638 from the London bookseller Robert Martin. None of the books was published earlier than 1610, and most date from the 1630s. Other Italian music that Jeffreys copied was not published until after 1660. That he continued copying in his later years is evident from the copies he made of three of Purcell's sonatas of 1683.
The Italian composer whose work Jeffreys seems to have studied most closely was Alessandro Grandi (i); 36 motets by him are in the British Library manuscripts. Other composers represented in these two sets of partbooks by more than ten works are Egidio Trabattone (22), Merula (21), Aloisi (15), Sances (12) and Facchi (11). It was undoubtedly through copying, studying and performing this music that Jeffreys acquired his understanding of seconda pratica techniques, which he subsequently applied to his own compositions (nine of which are settings of the texts of pieces in Add.31479 and Mad.Soc.G.55–9).