<p>This work provides a thorough examination of the sound transmission loss (STL) properties of double-walled sandwich microplates including a metamaterial honeycomb core and functionally graded porous piezoelectric (FGPP) facesheets subjected to thermal loading conditions. The structural and vibroacoustic response is investigated by using a size-dependent modified strain gradient theory (MSGT) combined with a first-order shear deformation theory (FSDT) to consider size effects and shear deformations. The geometrical dimensions of the honeycomb core are systematically changed to provide a broad variety of Poisson’s ratios, including positive, and negative (auxetic) values, allowing for metamaterial behavior. A modified power-law distribution model is employed to delineate the material property gradation in functionally graded piezoelectric facesheets, with four unique porosity distributions systematically altered to assess their influence on acoustic performance. The thermal effects are incorporated using a general thermomechanical coupling that considers uniform and nonuniform temperature gradients and nonlinear thermal distributions in the thickness direction. The governing equations are developed using Hamilton’s principle, and the solution is achieved analytically and confirmed against literature. Parametric investigations show that electric voltage, acoustic cavity depth, temperature gradients, length scale features, honeycomb cell structure, and power-law index factors significantly influence STL performance of the microplate system.</p>