Jack Cooper’s Modern Nature expanded on the last album, The Heat Warps, to include the U.K. improvisatory guitarist Tara Cunningham. Their dual guitar play was loose and intuitive, softening the lines drawn by the band’s motorik sound and adding an unpredictable but well-thought out element to its songs. Now, the two of them extend their conversation in Sleeves, in this quiet but riveting concoction of acoustic guitar and voices.