M4A is the audio file Apple devices produce: iPhone Voice Memos are saved as M4A, and so are tracks bought from iTunes. It is an MPEG-4 container that usually holds AAC audio, a codec that sounds as good as MP3 at a lower bitrate, but plenty of car stereos, older MP3 players, podcast hosts and voice upload forms still expect MP3. This page opens with MP3 as the output and 192 kbps as the bitrate, so you drop the M4A files and convert. Because AAC is already lossy, converting to MP3 is a second round of compression. Choose a bitrate at or above the original to avoid audible loss: iTunes purchases are 256 kbps, so 256 or 320 makes sense for music, while a voice memo is fine at 128. Some M4A files hold Apple Lossless instead of AAC, and those convert cleanly at any setting. Files bought with copy protection, such as Apple Music downloads, cannot be converted by any tool and will be marked as failed. Album artwork embedded in the M4A is not carried over, only the audio. Everything runs in your browser through FFmpeg compiled to WebAssembly. For uncompressed output use M4A to WAV Converter, and to transcribe a voice memo use Speech to Text Audio Transcriber.
The split that matters most in audio formats is lossy against lossless. A lossless format such as WAV or FLAC stores enough information to reconstruct the original waveform exactly, which is why WAV files run around 10MB per minute of stereo CD quality audio. Lossy formats such as MP3, OGG Vorbis, AAC, and Opus use psychoacoustic models to discard sound that listeners are unlikely to perceive, typically hitting a tenth of the size for audio most people cannot distinguish in normal listening. The important consequence is that the discarded information is gone permanently. Converting MP3 to WAV inflates the file back to full size without recovering anything, and each further lossy conversion compounds the loss, an effect known as generation loss. The practical rule is to keep a lossless master and export lossy copies from it, rather than converting between lossy formats repeatedly. Bitrate is where quality and size are actually traded. At 128 kbps most listeners cannot reliably distinguish MP3 from the source on laptop speakers or earbuds, though the gap becomes audible on good headphones with dense music. At 256 to 320 kbps MP3 is effectively transparent for nearly everyone. OGG Vorbis reaches similar quality at a somewhat lower bitrate, because it is a newer design than MP3, and Opus does better still, which is why messaging apps and voice chat use it. Sample rate and bit depth apply to the uncompressed side. This tool writes WAV at 16 bit and 44.1 kHz, the CD standard, which is the format editing software and mastering workflows expect, and which every audio application on every platform can open without conversion.