[Stream Recorder]

Some FLV and MP4 video files use AAC audio tracks. Instead of converting FLV and MP4 to M4A, M4V you can demux or extract AAC from FLV, MP4 to avoid recompression (and quality loss).

Software that can demux / extract AAC tracks from FLV and MP4 files

Replay Media Catcher 4 (http://all-streaming-media.com/record-video-stream/Replay-Media-Catcher-HTTP-RTMP-FLV-MP3-ripper-recorder.htm) (Shareware/Windows) - allows to extract MP3 or AAC audio tracks and save them as MP3 or M4A/M4B audio files.
FLV Extract (http://moitah.net/#FLVExtract) (Freeware/Windows) - allows to extract audio tracks and save them as MP3, AAC, Speex, or WAV (PCM)
Avidemux (http://avidemux.org/) (Freeware/Windows)
DSM Converter (from Media Player Classic) (http://sourceforge.net/projects/guliverkli/) (Freeware/Windows)

[Stream Recorder]

How to extract AAC autio tracks from FLV or MP4 video files with freeware command-line ffmpeg (Windows, Linux, Mac OS X)

First of all find out what type of audio is used in the video file:
ffmpeg -i inputfile.flv
You will get something like
Stream #0.1: Audio: AAC, 44100 Hz, stereo, s16, 128 kb/s

Now you know that AAC audio is used (or MP3, or Speex, or Wav,...), so use the following command:
ffmpeg -i input.flv -vn -acodec copy output.m4a
or
ffmpeg -i input.flv -vn -acodec copy output.m4b

[Stream Recorder]

Replay Media Catcher (http://all-streaming-media.com/record-video-stream/Replay-Media-Catcher-HTTP-RTMP-FLV-MP3-ripper-recorder.htm) 4 uses ffmpeg for extracting AAC audio tracks from FLV files to M4A and M4B. If you don't like command line tools, you can use RMC 4 instead:

http://img401.imageshack.us/img401/3606/replaymediacatcheraudio.jpg

How to convert FLV or MP4 file to M4A losslessly using Replay Media Catcher after downloading FLV or MP4 file automatically:

Open Replay Media Catcher.
Start Recording
Select the "Audio Extraction (Extract AAC to M4A)" conversion preset
Navigate to a web-site you want to record FLV stream from
HTTP and RTMP .FLV streams will be downloaded/captured automatically. Then AAC audio tracks will be extracted to M4A files automatically.


How to convert FLV or MP4 file to M4B losslessly using Replay Media Catcher after downloading FLV or MP4 file automatically:

Open Replay Media Catcher.
Start Recording
Select the "Audio Extraction (Extract AAC to M4B)" conversion preset
Navigate to a web-site you want to record FLV stream from
HTTP and RTMP .FLV streams will be downloaded/captured automatically. Then AAC audio tracks will be extracted to M4B files automatically.


How to convert any FLV or MP4 file to M4A losslessly with Replay Media Catcher 4:

Open Replay Media Catcher.
Select the "Audio Extraction (Extract AAC to M4A)" conversion preset
Go to "Tools" -> "Convert ..."
Select FLV files from which you want to extract AAC audio tracks
(CTRL key can help to select multiple files) and hit the "Open" button.
AAC tracks from the selected FLV file(s) will be extracted into M4A file(s)


How to convert any FLV or MP4 file to M4B losslessly with Replay Media Catcher 4:

Open Replay Media Catcher.
Select the "Audio Extraction (Extract AAC to M4B)" conversion preset
Go to "Tools" -> "Convert ..."
Select FLV files from which you want to extract AAC audio tracks
(CTRL key can help to select multiple files) and hit the "Open" button.
AAC tracks from the selected FLV file(s) will be extracted into M4B file(s)

[Sophialing]

As for me, the only thing I know is how to convert Apple Music M4P files to M4A, MP3, MP3 with ease so that I can listen to Apple Music on wanted devices and platforms.
Generally speaking, Apple Music are protected by DRM technology with encoded M4P format, which limits Apple Music lovers to play them on favorite Apple Music files. But free music converter for mac (https://www.drmare.com/drm-audio-converter-for-mac) can help you remove DRM protection from Apple Music and convert them to MP3, M4A, M4B, FLAC, etc. for offline playback on any device.

See More：How to Convert Apple Music to MP3 (https://www.drmare.com/apple-music/convert-apple-music-m4p-to-mp3.html)