There's an important distinction buried in the name "AAC file" that trips a lot of audio software up: a raw .aac file is a bare ADTS-framed AAC bitstream with no surrounding container, while an M4A file is that same AAC audio wrapped inside an MP4-style container that carries metadata, chapter markers, and seek tables. Plenty of recording apps, voice memo tools, screen-recording software, and some exported streams produce the bare, unwrapped version — a raw .aac file — rather than the more common M4A. This tool is built specifically for that raw stream.
The practical problem raw AAC creates is that a surprising number of audio editors and DAWs either can't open it directly or handle it unreliably, even though they're perfectly happy with M4A or WAV. Without the container's framing information, some software can't locate the start of the stream correctly or misjudges the duration. Converting to WAV sidesteps the whole issue: WAV is about as close to a universally-accepted format as audio editing software gets, so once the AAC is decoded into a WAV file, compatibility stops being a question.
It's worth being precise about what this conversion does and doesn't accomplish. Decoding raw AAC to WAV unpacks the compressed bitstream into uncompressed PCM samples — it does not and cannot restore any audio detail the original AAC encoder discarded when the file was first created. Whatever quality ceiling that original encode set is the quality ceiling you get here too, just stored without further compression. What changes is the container and the compression state, not the underlying fidelity.
The decode runs entirely through FFmpeg compiled to WebAssembly inside your browser, so voice memos, screen-recording exports, or other sensitive audio never need to be uploaded anywhere to become editable.
If your file is actually an M4A rather than a bare .aac stream, the dedicated M4A to WAV tool handles that container correctly.
