M4A to WAV.

No, and this is worth being direct about because the file size makes it look like it should. The WAV is a faithful copy of the decoded M4A — every frequency the AAC encoder threw away is still missing, it is just now stored inefficiently. What you gain is compatibility and the ability to edit without re-encoding, which are real, but the audio is unchanged.

Because WAV stores every sample at full size and AAC stores a compressed description of what you would hear. Around 10 MB a minute against roughly 1 MB is the normal ratio. That extra space is empty in the sense that matters — no new detail arrived with it.

Yes, that is the right order. Editing an already-lossy file and re-encoding it adds a generation of loss at every save; working in WAV and encoding once at the end adds exactly one. If the source is an M4A you are starting one generation down regardless, so it is worth keeping the rest of the chain lossless.

No, and that is the main reason to use this rather than a site that asks you to wait for an upload. The M4A is decoded, re-written as WAV and handed back entirely inside this browser tab — open the network panel and watch it stay empty. There is no server behind the page, which means an unreleased promo is safe to drop on it and nothing is sitting in somebody's bucket afterwards.

Yes. Drop as many files as you like, set the format once, and the whole queue is converted with a report for each file — what it came from, what it went to, and how the size compares. The results come back as a single archive rather than as thirty separate downloads, which is what you want when the next step is dragging a folder into Rekordbox.

Yes. Artist, title, album, album artist, genre, year, track number, label, remixer, composer, BPM, key, ISRC, comment and the cover art are all read off the M4A and written into the WAV, so a converted folder arrives in Rekordbox named rather than as a wall of filenames. The one thing that does not travel is anything a DJ program keeps in its own private frames — Serato's cue points and beat grid live in an undocumented block that nothing outside Serato reads, and no converter can carry them.

Up to 800 MB per file, which is about 75 minutes of 24-bit stereo audio — a full recorded set fits comfortably. Past that it is the browser's own decoder that gives up rather than the tool.

The lossless formats are written here, and verified: every test file was decoded again by an independent decoder and compared sample by sample against the original, with zero differences. On real records the FLAC comes out at about 67% of the uncompressed size, which sits between the standard encoder's default setting and its slowest one. MP3 uses the reference encoder that has been the answer to that problem for twenty-five years, because a homemade one would not make bigger files, it would make worse-sounding ones.