Measurements of microbubble (MB) shell parameters is a challenging task because of the nonlinear dynamics of MBs. Shell parameter estimations that are typically based on solving linear models will generate inaccurate results, especially at higher pressure excitations. These approaches also often ignore the analysis of sound speed which provides useful information about the bulk modulus of the medium. In addition, the effect of MB-MB interaction is neglected. In this work, the attenuation and sound speed of monodisperse MB populations with mean diameters of 4 to 6 micron and peak concentrations of 1000 to 15000 bubbles/ml are measured for a pressure range of 10 to 100 kPa. The subharmonic pressure threshold of the solution was measured by narrowband excitations spanning from 1 to 4 MHz. The subharmonic generation pressure threshold was used to estimate an initial guess for shell viscosity and surface tension. The experimental results were fitted using numerical simulations of the Marmottant model and our recently developed nonlinear model for attenuation and sound speed. The effect of MB-MB interaction was also implemented using simulations of a lattice of interacting MBs (fitted to the measured sizes of MBs) to take into account the effect of concentration.