Sonoluminescence (SL) observed in the cavitation of water may be explained by the Planck theory of SL that treats the bubbles as collapsing miniature masers having optical waves standing in resonance with the dimensions of bubble cavity. Microwaves are created from the Planck energy of the standing waves provided the bubble wall may be treated as a perfect blackbody surface. In the ultraviolet, liquid H₂O is strongly absorbent and the bubble approaches a Planck blackbody enclosure. The micrwaves are created at frequencies proportional to the bubble collapse velocity and are absorbed by the dipoles of the H₂O and other bubble wall molecules. Intense electric fields develop as the liquid H₂O bubble wall undergoes dielectric polarization. By this theory, free electrons are created in SL as the electric fields breakdown; the presence of free electrons is required if any magnetic field effect is to be observed in SL. Both local and global magnetic effects on SL are described. The local effect is based on the magnetic pressure due to the electrons moving as currents inside the bubble. The global effect is an accumulation of local effects at the voids throughout the liquid H₂O causing a reduction in the bulk modulus. Numerical solutions of the Rayleigh-Plesset (R-P) equation are presented that show the effect of applied magnetic field on SL to be the global effect causing a reduction in the bulk modulus. Consistent with the Planck theory of SL, the R-P simulations show the suppression of SL intensity with magnetic field to be parabolic and the SL intensity to be linear with collapse velocity.