The novelty of this research is to consider simultaneously forced vibration response of a nine-layer sandwich microbeam with homogeneous core reinforced by nanocomposite and piezo-magneto-electric layers under electric and magnetic fields in x and z directions as a self-sensing mass sensor at the end core layer. Also, the present study, the natural frequencies based on two methods, including the separation of variables for different B.C.’s and Navier’s method for S-S case are obtained. In this study, the effects of various parameters such as an added mass at the end of core, different Young's modulus to the density ratio, different properties of core, volume fraction of CNT, thickness ratio, aspect ratio, length of each layer to length of the core ratio, various distribution of carbon nanotube (CNT), material length scale parameter, electric and magnetic fields, input voltage on the frequency response function (FRF) amplitude are investigated. It shows that the natural frequency for C-C is higher than the other cases (C-S, S-S, C-F), because the stiffness of structures increases. Moreover, it shows that the natural frequency based on modified couple stress theory (MCST) is higher than classical theory (CT), because the material length scale parameter enhances the stiffness of a sandwich beam. It illustrates that the natural frequency for the FG-X distribution is higher than for other distributions, because the stiffness of the structure at the furthest distance from the neutral axis. It shows that with an enhance in the length ratio and aspect ratio, the natural freqeucny reduces, because the structure becomes softer. It considers that the effect of electric and magnetic fields is simulteneously higher than the other cases, because the stiffness of the micro beam enhances. The findings of this study can be crucial in the design of the engineering and medical industries.