Catalyzes the oxidative demethylation of sarcosine to yield glycine, hydrogen peroxide and 5,10-methylenetetrahydrofolate. H2O + O2 + sarcosine = formaldehyde + glycine + H2O2 Binds 1 FAD per subunit. Binds 1 FMN covalently. Heterotetramer (SoxA, SoxB, SoxG and SoxD). Belongs to the SoxB family.