macOS includes a tool called afconvert which describes itself this way in the man page.
Audio File Convert will convert a source audio file to a new audio file with the specified file and data types
However, that's the entire man page. What are the command line options required and supported by afconvert? What are some basic examples of how to convert audio files back and forth?
Audio File Convert exports to 20 audio file formats and a multitude of codecs, all of which are shown with the -hf option.
File formats: AIFC AIFF NeXT Sd2f WAVE RF64
The original formats for storing audio files were all uncompressed because these are easy to work with, and can be played by even very early CPUs. However, uncompressed audio files are very large, easily going up to 10 MB/minute. These formats all share similar codec options like I8 BEI16 BEI24 BEI32 BEF32 BEF64. Smaller numbers give smaller files, but larger numbers give higher quality. If you're not sure what to choose, go with BEI32 or LEI32.
BE big endianLE little endianI integerUI unsigned integerF floating point8/16/24/32/64 bits per sampleYou should only need to specify these options for these older uncompressed formats.
File formats: 3gpp 3gp2 adts ac-3 amrf m4af m4bf caff ec-3 flac mp4f
Not every codec is supported by every file format, but you can use this as a guide to get started choosing your codec. Once you've chosen a codec, you can look at the output of afconvert -hf to pick a suitable container file format.
aac MPEG-4 Low Complexity AACaace MPEG-4 AAC Enhanced Low Delay (without SBR)aacf MPEG-4 AAC Enhanced Low Delay (with SBR)aacgaach MPEG-4 High Efficiency AACaacl MPEG-4 AAC Low Delayaacp MPEG-4 High Efficiency AAC Version 2ac-3 AC-3alac Apple Losslessalaw aLaw 2:1drms Apple DRMec-3flac Free Lossless Audio Codecpaacpac3pec3samrsawbQDM2 QDesign2 MusicQDMC QDesign MusicQclp QUALCOMM PureVoiceQclq Qualcomm QCELP(Note: I have excluded the MPG formats such as MPG3/MP3 from this list because they just do not work.)
Also see this answer for more info on the various codecs and where they are defined.
Once you've chosen a file format and codec, you run afconvert like this:
afconvert -f FORMAT -d CODEC SOURCEFILE [-o DESTINATIONFILE] It is necessary to specify both the file format and the codec, otherwise an error is shown. If you omit the -o option, afconvert will pick a name and extension based on the other inputs.
If you have an audio file called MyAudio.m4a you could try these commands.
# AIFF afconvert -f AIFF -d BEI32 MyAudio.m4a -o MyAudio.aiff # WAVE afconvert -f WAVE -d LEI32 MyAudio.m4a -o MyAudio.wav # Apple Lossless afconvert -f m4af -d alac MyAudio.m4a -o MyAudio.m4a # FLAC afconvert -f flac -d flac MyAudio.m4a -o MyAudio.flac # AAC afconvert -f mp4f -d aac MyAudio.m4a -o MyAudio.mp4 
2wav() { afconvert -f WAVE -d LEI32 ${1} ${1}.wav ; } for f in *.aif ; do afconvert -f WAVE -d LEI32 "$f" -o "${f%.aif}.wav"; done alias 2wav="afconvert -f WAVE -d LEI32 \$1 \$1.wav". I've run into an important caveat with afconvert.
the help text gives many options, for instance --bitrate.
These options are codec dependent, and not all options work for all codecs. I haven't been able to find any definition of what codecs support which options, i've been going by trial and error.
The error message when using an apparently-unsupported option for the codec is:
Error: Couldn't set audio converter property ('!dat') 
afconvert --help. Many (not all, but afconvert is one of them) command line programs will print some documentation in response to the--helpoption.