  We investigate the acoustic normal modes ("phonons") of a 1D microfluidic
droplet crystal at the crossover between 2D flow and confined 1D plug flow. The
unusual phonon spectra of the crystal, which arise from long-range hydrodynamic
interactions, change anomalously under confinement. The boundaries induce
weakening and screening of the interactions, but when approaching the 1D limit
we measure a marked increase in the crystal sound velocity, a sign of
interaction strengthening. This non-monotonous behavior of the phonon spectra
is explained theoretically by the interplay of screening and plug flow.
