Metallic foams of A356 alloy with a uniform porosity of 75-85% were produced with the melt foaming process. The microstructure inside the foam cell walls was experimentally studied. It is found that the microstructure is greatly different from the as-cast structure of the base alloy. It is believed that the thickening process with calcium, the adding of foaming agent and the mixing process during the production process all play important roles on the solidification microstructure of the cell walls. The morphology and grain size of primary α-phase, the amount and morphology of eutectic silicon, the distribution and size of CaSi2Al2 and residual titanium hydride particles were studied. The sound absorption coefficient of the alloy foams was measured. Two methods, drilling small holes and compressing the foam, have been developed to improve the sound absorbability of the alloy foam in low and middle frequency ranges.