This paper illustrates the development of a novel sandwich panel for vibroacoustic applications by using novel materials as passive damping mechanisms. A flax-fibre composite is adopted for lattice core and skins. Carbon-nanotube reinforced viscoelastic inserts are embedded in critical locations within the lattice to further increase its vibration damping and potentially increase its strength. Restrictions to the mass added by the inserts are imposed to preserve the lightweight advantage of the panel. Quasi-static analyses are carried out to guarantee good-quality structural performances. The flatwise compression characterisation of bare- and reinforced cores shows that both configurations outperform comparable benchmark cases in terms of normalized stiffness and strength. The reinforced configuration presents 24.5% higher normalized strength compared to the bare panel. Two different types of vibration tests are performed. Results of out-of-plane vibration transmissibility tests show that the bare configuration outperforms that with inserts due to Coulomb dry friction effects. The CNT reinforced panel offers the highest viscoelastic damping in free vibrations conditions. © 2023 International Committee on Composite Materials. All rights reserved.