"MKV to MP4", "FLAC to MP3", "M4A to MP3" — chances are you typed one of those into Google search at some point, because media file formats can be a pain. Some are way too space-consuming, like FLACs, which can be 10 times bigger than equivalent MP3s. Others are unsupported by your Mac's built-in video player, like MKV. No matter the specific case, you're going to need an audio or video converter to transform the file into a friendlier, more widely used format, which is typically MP4 for video and MP3 for music.