If a peach seems to have smooth, blemish-free skin but is a bit brown inside, the usual reason is the farmer didn't let the fruit ripen before picking it from the peach tree.  Peaches that are going to be exported are typically harvested before they're completely ripe so they don't get bruised and squashed during transportation to local grocery stores. These peaches are sometimes frozen too. So long as there are no other indicators of spoilage, a brown hue inside the peach won't affect its flavor.