AACGain: Normalize M4A and AAC Files the Lossless Way

MP3Gain stops at the edge of the MP3 format — but its engine has a sibling. AACGain, written by Dave Lasker, is a modified version of the MP3Gain command-line engine that applies the exact same lossless trick to AAC audio: the files inside your iTunes-era library, ending in .m4a or .mp4. It analyzes loudness with the ReplayGain algorithm, then rewrites the gain value stored inside each AAC frame — no decoding, no re-encoding, and the change can be undone. This guide covers how it relates to MP3Gain, how to actually run it (including through the familiar MP3Gain window), and the real limits you need to know first.

Same engine, different format

The relationship is literal, not marketing. AACGain began as a fork of the MP3Gain back-end code, coordinated with MP3Gain’s author Glen Sawyer and announced on the official MP3Gain site in January 2005. Where MP3Gain’s engine only understands MP3 frames, Lasker’s version parses AAC frames — both raw AAC streams and, far more commonly, AAC audio stored inside an MP4 container, which is what .m4a and .m4b files are. Everything upstream of the frame parsing is shared heritage: the same ReplayGain loudness analysis, the same 89.0 dB default target, the same Track Gain and Album Gain modes, and the same adjustment granularity of 1.5 dB steps, because AAC frames carry a per-frame gain field just as MP3 frames do.

That shared mechanism is why AACGain deserves the word “lossless” in the same qualified sense we use for MP3Gain itself — see Is MP3Gain Lossless? for the full explanation. The compressed audio data is never touched; only the per-frame gain instructions and the undo/analysis information change. If you apply a gain change and dislike the result, the undo data lets you roll the file back, exactly as with MP3Gain. What AACGain does not do is make an AAC file play louder in a player that ignores the frame gain — but unlike tag-only methods, every decoder reads frame gain as part of normal playback, so the adjustment travels with the file to any device.

How to run it: three realistic routes

Route 1 — the MP3Gain GUI trick (the officially documented one). AACGain is a command-line program, but Lasker wrote it specifically so it would work with the existing MP3Gain GUI without much trouble. The official MP3Gain site documents the procedure: download the latest MP3Gain, then download AACGain, unzip aacgain.exe, rename it to mp3gain.exe, and move it into the MP3Gain program folder, replacing the existing backend. From then on, the window you already know analyzes and adjusts AAC files (.m4a or .mp4) exactly as it did MP3s. Keep a copy of the original backend if you want to switch back.

Route 2 — a dedicated front-end. A small Windows front-end called iGain was written to wrap aacgain.exe in a simple graphical window, and the cross-platform QMP3Gain front-end (covered in our MP3Gain for Mac guide in the context of MP3) can also drive the MP3Gain-family backends, including AACGain. These are community tools from the same era as AACGain itself, so expect a functional, dated interface rather than a polished modern app.

Route 3 — the command line. Because AACGain shares MP3Gain’s engine, its switches mirror the ones in our MP3Gain command line guide: analyze first, apply Track or Album Gain, and use the undo switch if you change your mind. One practical wrinkle: because of AAC patent licensing, AACGain was distributed as source code through the MP3Gain project on SourceForge rather than as an official binary release, so the prebuilt aacgain.exe copies in circulation come from long-standing third-party mirrors and project builds. Download it from a source you trust, and — as with any file-level tool — run it on copies of your library first.

The limits, stated plainly

• DRM files are off-limits. The official MP3Gain site states it directly: AACGain will not work on DRM-encoded files — the protected .m4p purchases from the early iTunes Store era. Files you ripped or encoded yourself work fine, and modern iTunes/Apple Music purchases have been DRM-free for years, so this mainly affects genuinely old libraries.
• It is a legacy tool. Like MP3Gain, AACGain has not been actively developed in years. It remains widely used because the format problem it solves has not changed, but there is no support desk and no promise of updates for future operating systems.
• Apple’s own ecosystem plays by different rules. Inside Apple Music and iTunes, volume leveling is handled by Sound Check metadata, not by frame gain. If your listening happens entirely inside Apple’s apps, read iTunes Sound Check vs MP3Gain before processing anything — applying both systems to the same files double-adjusts them.
• 1.5 dB steps still apply. Adjustments land within 0.75 dB of the target rather than exactly on it, the same trade-off as MP3Gain, for the same structural reason.

Track Gain or Album Gain for AAC files?

The same judgment call applies as with MP3s, because it is the same analysis underneath. Use Album Gain for anything released as an album — it preserves the deliberate loudness differences between tracks while leveling the album against the rest of your library. Use Track Gain for singles, compilations you assembled yourself, podcasts, and audiobook files, where each file should stand at the reference level on its own. The reasoning, including the edge cases around classical music and gapless albums, is laid out in Track Gain vs Album Gain; every word of it applies to AACGain unchanged. Two AAC-specific notes: audiobooks in .m4b form are just AAC in an MP4 container, so they process like any other file, and because only frame gain values and analysis tags change, chapter markers and cover art stored in the container are left as they were. And if you are mixing AACGain-processed files into a library that also lives in Apple Music, re-read the limits section above about Sound Check before you sync anything.

Should you use it?

If your library is a mix of MP3 and M4A and you want one consistent, player-independent volume level baked into the files themselves, AACGain is the only tool that extends MP3Gain’s proven, reversible method to the AAC half of that library. If instead your AAC files live and die inside Apple Music, Sound Check (or simply leaving the files alone) is the less invasive answer. And if a file is in some other format entirely — FLAC, WAV, OGG — neither program applies; our format support overview maps each format to the tool that actually fits it.