Millimeter to centimeter size light-metallic shells are of interest as investigative tools in the inertial-confinement-fusion community. Such shells are typically filled at room temperature with pure or mixed hydrogen isotopes at high pressure. At low temperatures, the hydrogens condense forming a solid or liquid in equilibrium with the vapor. The sound speed of gaseous deuterium (1% He-3) in a closed aluminum shell has been investigated using a cavity/shell resonance method. An anomalous decrease in the measured sound speed is observed as the saturation pressure P[inf s] (about 15 atm at 38 K) is approached during a nearly constant-volume reduction in temperature. This effect is explained qualitatively in terms of a reduced acoustic admittance at the shell wall due to a precondensed liquid layer at pressures below P[inf s] [J. B. Mehl and M. R. Moldover, J. Acoust. Soc. Am. 77, 455--465 (1982)]. Comparison is made with precondensation phenomena observed by various authors for single-component gases in experiments performed at constant temperature.