This paper reports accurate measurements of the speed of sound in compressed pure nitrogen and a synthetic ternary mixture of nitrogen, oxygen, and argon with a composition close to that of natural dry air. The data have been measured by a double-path-length pulse-echo technique and cover the temperature ranges between 275 and 400 K for nitrogen and between 240 and 420 K for the dry air mixture with pressures up to 100 MPa. The expanded uncertainties (at the 0.95 confidence level) amount to 2.1 mK for temperature, SO ppm for pressure, 80 ppm for speed of sound in nitrogen, and 110 ppm for speed of sound in the dry air mixture.