iTunes is capable of reading, writing and converting between MP3, AIFF, WAV, MPEG-4, AAC and Apple Lossless (.m4a) files. To see this information go to Preferences → General tab → Import Settings. That means that the Apple media player does not support and cannot convert FLAC files. However, there are ways to deal with this obstacle.
What Is FLAC and Why Is It Not Supported by iTunes?
FLAC (Free Lossless Audio Codec) is a musical file format that ensures a high sound quality, as it retains all the audio information. That is why it is called a lossless format, unlike MP3, for example, which sacrifices the highest and lowest end of the range in favor of creating smaller audio files.
It is not supported by iTunes because of potential legal problems for Apple. FLAC is an open-source format; hence, it can be a target of patent trolls. So, Apple created its own lossless codec called ALAC, which offers the same sound quality.
Converting FLAC to ALAC
Converting FLAC to ALAC seems to be the first option to try for people who truly care about the sound. There will be no loss of quality in this case. To do this, you can use a free XLD app, or a paid alternative, dBpoweramp. The latter allows converting FLAC to ALAC by right-clicking necessary files, which are then automatically added to iTunes.
Note that ALAC files look like AAC files in the Finder sharing the same extension.
Converting FLAC to Lossy Formats
Users can also convert FLAC to MP3, for example, which is a more common format. MP3 files also take up to six times less space than FLAC files. Keep in mind that reverb, cymbals, and guitars are usually the sounds affected the most in the process of compression to MP3 files.
If you need to convert a few files, then you can use online file converters without a need to download any software. It can be zamzar.com, online-convert.com, convertfiles.com, aconvert.com, coolutils.com, or media.io.
There are several apps that can convert FLAC files to MP3 files. You can use free tools like fre:ac, MediaHuman Audio Converter, and Switch, which has a paid version that does not limit you to converting only five files at a time.
Bigasoft FLAC Converter is another paid option that is also a FLAC editor. It provides a possibility to cut out audio parts or join several FLAC files into one file.
Using other Media Players That Support FLAC
There are also alternative media players available for macOS and iOS that allow users to listen to FLAC files without converting. For example, you can look for Clementine, which is a free and open-source app. Then, you can get FLAC Player, MediaConnect directly from the App Store.
The list of media players that support the FLAC format also includes Roon, Golden Ear, TuneShell, and Fluke. The latter is free and can run unmodified FLAC files in iTunes. However, it has not been updated for some time, so it may not work properly with the latest versions of the macOS.