Polydimethylsiloxane (PDMS) nanocomposite films for underwater sound absorption applications are presented. Films were fabricated by a spin coating method and characterized using XPS, FTIR, TGA, SEM, tensile testing, and elastic cold-neutron scattering. The XPS results reveal the addition of surfactant and carboxyl-functionalized multi-walled carbon nanotube (MWCNT-COOH) increased the components of C and O atoms by 4.12% and 2.81%, respectively, compared to the native PDMS film. The addition of MWCNT-COOH and surfactants to the PDMS also significantly improved the thermal stability (700 degrees C) of the nanocomposite material compared to pure PDMS. Tensile property measurements of PDMS nanocomposites indicate that the addition of 2 wt% of MWCNT-COOH significantly improves the elastic modulus by 48% compared to the pure PDMS. The addition of surfactant nonetheless decreases the Young's modulus by 34% compared to pure PDMS, due to changes in the microstructure of the PDMS matrix. The advantage of the decreased Young's modulus is to avoid acoustic impedance mismatch. The surface morphology detected by SEM indicates that the surfactant can improve the dispersion of MWCNT-COOH in the PDMS films. Further, the elastic cold-neutron scattering results demonstrate the diffusive motions near the glass transition temperatures (T-g) of the nanocomposite films, providing in-depth details of the dynamic alignment of the inclusions within the material matrix. The PDMS/MWCNT-COOH/surfactant nanocomposite exhibited a sound absorption coefficient of 0.25, which was far higher than the PDMS control materials. This positive result indicates these PDMS nanocomposite films have promising future underwater sound absorption applications.