Format guide

Slow Down AAC and M4A Online

Slow down AAC and .m4a files in your browser with speed, reverb and bass controls. iTunes purchases, Voice Memos, DRM files that will not decode, MP3 export.

Open the free editor: upload MP3, WAV, AAC, OGG or FLAC, adjust speed, reverb, bass, tone, width and space, then export MP3 or a ZIP of many tracks. Everything runs in your browser.

AAC is the format Apple built its ecosystem around, which means it is the format of iTunes Store purchases, iPhone Voice Memos, screen recordings and most video soundtracks. The usual extension is .m4a, sometimes .aac or .mp4. The editor above decodes these in your browser, slows them from 0.50x to 1.50x with or without a pitch drop, adds reverb, bass, tone and width, and encodes an MP3 locally. Nothing is uploaded.

This page covers what is particular to AAC: which .m4a files decode and which do not, why Safari handles AAC better than it handles some other formats, how iTunes purchase bitrates hold up when slowed, and the export. The headline caveat comes first because it saves the most frustration: DRM-protected files will not decode, and no browser tool can change that. Details are below.

For the controls themselves, start at the slowed reverb maker. iPhone users should also read the iPhone page, since AAC is the format most of your files will be in. If you have a lossless alternative, the WAV page explains why it slows more cleanly.

Which .m4a files work

The .m4a extension is an MPEG-4 container. It can hold AAC audio, which the editor handles, or Apple Lossless (ALAC), which browsers also generally decode, or protected AAC with FairPlay DRM, which nothing outside Apple's players can decode.

The simple test is to load the file. If the browser cannot decode it, the editor tells you rather than producing silence.

DRM-protected files will not decode

Say it plainly because it is the most common reason an .m4a fails here. Apple Music downloads and older iTunes purchases with the .m4p extension are encrypted with FairPlay. Only Apple's own apps on authorised devices can play them. A browser cannot decode them, so this editor cannot slow them, and neither can any other browser tool or desktop converter that respects the encryption.

If a file you want to slow is Apple Music content, you need a different copy. Options: buy the track on the iTunes Store, where purchases since 2009 are DRM-free AAC at 256 kbps; buy it from Bandcamp or another store that sells unprotected files; or rip it from a CD you own. There is no setting in this editor that works around DRM, and the site will not suggest tools that strip it.

Older iTunes purchases in .m4p form can sometimes be upgraded to DRM-free versions through Apple's own services, depending on your account and region. That is between you and Apple.

Safari and AAC

Safari's media stack is built around AAC, so .m4a files are the most reliable sources on iPhone, iPad and Mac. Where FLAC and Ogg support in Safari varies by version, AAC decoding is consistent. If you are working on an iPhone and have the choice of formats, AAC at 256 kbps is the safe pick, and the iPhone page covers how to get files from the Files app into the editor and where the export lands.

Chrome, Edge and Firefox also decode AAC on all platforms, so an .m4a is one of the few formats you can count on everywhere.

How iTunes-quality AAC holds up when slowed

AAC at 256 kbps is a very good lossy source. It is more efficient than MP3 at the same bitrate, so a 256 kbps AAC sits close to a 320 kbps MP3 in how much high-frequency detail survives. Slowing stretches encoder artifacts into audibility just as it does with MP3, but at 256 kbps there is little to stretch. You can slow to 0.75x without the swirl you would get from a 128 kbps file.

Lower-bitrate AAC, as found in some streaming rips and older files, behaves like low-bitrate MP3: fine at 0.90x, swirly at 0.78x. Darken the Tone and slow less. The MP3 page has a bitrate table that maps reasonably well onto AAC, shifted one step more forgiving.

Voice Memos and recorded speech

Voice Memos on iPhone export as .m4a, as do many recorder apps on Android. These slow down well. For speech you almost always want Preserve pitch on, so the voice keeps its natural register while the pace changes. A lecture at 1.25x or an interview you need to transcribe at 0.85x both stay clear while you listen. Preserve pitch changes the live preview only; exported files keep the speed-linked pitch, so for an in-key file use Audacity's Change Tempo. Leave Reverb at 0 and Bass and Tone at 0 unless the recording is thin or harsh. The preserve pitch page explains where the time-stretch starts to warble; for speech, stay between 0.80x and 1.40x.

Recordings made in noisy rooms slow down with the noise intact. The editor has no noise reduction.

Video files

Audio in .mp4 and .mov files from a phone is usually AAC. The editor's file picker accepts audio types, and some browsers will decode the audio track from a video container when you pick it. If a video file fails to load, extract the audio first with a free tool or a DAW, then load the .m4a or .wav. The editor does not export video; for a slowed version of a video you would export the MP3 here and reattach it to the picture in a video editor. The video guide walks through that.

The export

Output is MP3 at 96 kbps, 32 kHz, encoded in a web worker in your browser. There is no AAC, M4A, ALAC or WAV output and no higher MP3 bitrate. An AAC source through this chain means two lossy stages before a video platform adds its own. With a 256 kbps source that is fine for YouTube, TikTok, Reels and Shorts, all of which re-encode uploads. For a lossless or distribution-grade file, find your settings here and reproduce them in a DAW from a WAV or ALAC source.

The exported file is named yourfile-remix.mp3. Album art and tags from the .m4a are not carried over; add them afterwards with a tag editor if needed. Batch exports arrive as a ZIP, and the bulk converter page explains the naming.