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I would say sound travels faster in metals than in rocks because rocks may be harder than metals but they don't have mobile electrons to pass sound wave through.
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That is definitely wrong.
Adding to what Gokul mentioned, metals are usually denser that rock and ceramics. Compare with steel's specific gravity of ~ 7.5-8, depending on alloying elements and composition.
Density of common minerals -
http://www.mininglife.com/Miner/general/Density.htm.
Specific Gravity Table For Ceramics, Metals & Minerals -
http://www.reade.com/Particle_Briefings/spec_gra.html
Generally metals are two or three time denser than minerals/ceramics (i.e. metal oxides). Also, surface minerals are likely to have porosity of several %.