The nonlinear coupling between circularly polarized electromagnetic (CPEM) waves and acoustic-like waves in a magnetoactive electron-positron-ion (e-p-i) plasma is studied, taking into account the relativistic motion of electrons and positrons. The possibility of modulational instability and its growth rate as well as the envelope soliton formation and its characteristics in such plasmas are investigated. It is found that the growth rate of modulation instability increases in the case that ωc/ω<1 (ωc and ω are the electron gyrofrequency and the CPEM wave frequency, respectively) and decreases in the case that ωc/ω>1. It is also shown that in a magnetoactive e-p-i plasma, the width of bright soliton increases/decreases in case of (ωc/ω)<1/(ωc/ω)>1 by increasing the magnetic field strength.