In this contribution, we discuss preliminary research results obtained within the framework of one of the ten individual research projects of the EU-MSCA-DN, “ActaReBuild”, which focuses on the development of acoustic resonators based on lightweight concrete using recycled aggregates. In particular, we examine using loose granules of flo-pak plastic as material to fill and absorb sound in the resonator cavity. The sound absorption of the granules was measured in a reverberant room and compared with the sound absorption properties of melamine foam, a common porous material of the same thickness. Also the absorption of a wooden resonator (1) filled with melamine and (2) filled with loose plastics was determined and compared to that of (3) an empty resonator. The results show that adding granules in the cavity leads to an increase in resonance frequency, likely due to their closed pores reducing the effective cavity volume and increasing stiffness. Also broadening of the absorption peak is observed.