I've used Fission (https://itunes.apple.com/th/app/fission/id549251391?mt=12) but there's possibly also Open Source/free options if you don't otherwise have use for Fission (I bought it to make ringtones, the batch convert is a bonus).
If you use iTunes, FLACTunes is very convenient and simple. It is a paid app ($0.99). Select the target format, drag and drop the files to convert into the window and they show up converted in your iTunes library.