Two semiconductor optical frequency combs, consuming less than 1 W of electrical power, are used to demonstrate high-sensitivity mid-infrared dual-comb spectroscopy in the important 3-4 μm spectral region. The devices are 4 mm long by 4 μm wide, and each emits 8 mW of average optical power. The spectroscopic sensing performance is demonstrated by measurements of methane and hydrogen chloride with optical multi-pass cell sensitivity enhancement. The system provides a spectral coverage of 33 cm-1 (1 THz), 0.32 cm-1 (9.7 GHz) frequency sampling interval, and peak signal-to-noise ratio of ∼100 at 100 μs integration time. The monolithic design, low drive power, and direct generation of mid-infrared radiation are highly attractive for portable broadband spectroscopic instrumentation in future terrestrial and space applications.