Catalyzes the reduction of free and protein-bound methionine sulfoxide to methionine. Isoform 2 is essential for hearing. [thioredoxin]-disulfide + H2O + L-methionyl-[protein] = [thioredoxin]-dithiol + L-methionyl-(R)-S-oxide-[protein] [thioredoxin]-disulfide + H2O + L-methionine = [thioredoxin]-dithiol + L-methionine (R)-S-oxide Binds 1 zinc ion per subunit. Monomer. Widely expressed. The disease is caused by variants affecting the gene represented in this entry. A nonsense mutation affecting exclusively mitochondrial isoform 2 is sufficient to produce hearing loss. Has a transit peptide. Belongs to the MsrB Met sulfoxide reductase family.