The authors present numerical results for a systematic parametric study of the effect of honeycomb core geometry on the sound transmission and vibration properties of in-plane loaded honeycomb core sandwich panels using structural acoustic finite element analysis (FEA). Honeycomb cellular structures offer many distinct advantages over homogenous materials because their effective material properties depend on both their constituent material properties and their geometric cell configuration. From these structures, a wide range of targeted effective material properties can be achieved thus supporting forward design-by-tailoring honeycomb cellular structures for specific applications. One area that has not been fully explored is the set of acoustic properties of honeycomb and understanding of how designers can effectively tune designs in different frequency ranges. One such example is the insulation of target sound frequencies to prevent sound transmission through a panel. This work explored the effect of geometry of in-plane honeycomb cores in sandwich panels on the acoustic properties the panel. The two acoustic responses of interest are the general level of sound transmission loss (STL) of the panel and the location of the resonance frequencies that exhibit high levels of sound transmission, or low sound pressure transmission loss. Constant mass honeycomb core models were studied with internal cell angles ranging in increments from −45 deg to +45 deg. Effective honeycomb moduli based on static analysis of honeycomb unit cells are calculated and correlated to the shift in resonance frequencies for the different geometries, with all panels having the same total mass. This helps explain the direction of resonance frequency shift found in the panel natural frequency solutions. Results show an interesting trend of the first resonance frequencies in relation to effective structural properties. Honeycomb geometries with smaller core internal cell angles, under constant mass constraints, shifted natural frequencies lower, and had more resonances in the 1–1000 Hz range, but exhibited a higher sound pressure transmission loss between resonant frequencies.