AAC (Advanced Audio Coding) is the codec Apple, YouTube, and most modern recording and streaming software settled on as a default, and for good technical reason — AAC was standardized well after MP3 and takes advantage of later psychoacoustic modeling improvements, so it genuinely sounds better than MP3 at the same bitrate. Converting a raw AAC file to MP3 is, strictly speaking, a step backward in compression efficiency, not an upgrade.
This is the exact same codec relationship behind MP4-to-MP3 conversion, since an MP4 video's audio track is almost always AAC under the hood — the only difference here is that you're starting from a bare .aac audio file instead of a video container. Whether the AAC arrives wrapped in MP4 or as a standalone stream, converting it to MP3 means trading away AAC's efficiency advantage.
People do it anyway for one consistent reason: MP3 compatibility beats AAC's efficiency in a lot of real-world playback situations. Older car stereos, cheap standalone MP3 players, some DJ software, and various non-Apple applications and embedded devices support MP3 without question while having spotty or no AAC support outside Apple's own ecosystem and major streaming platforms. This is a genuine trade-off, not a free upgrade — you're giving up some of AAC's technical efficiency specifically to gain MP3's much broader hardware and software reach.
The conversion itself is a lossy-to-lossy re-encode: the AAC stream is fully decoded to raw PCM, then that PCM is re-encoded through libmp3lame at quality level 2, a variable-bitrate setting averaging roughly 190kbps that's widely documented as transparent in listening tests. At that setting the practical audible difference is usually negligible, even though the underlying codec math is technically running in the less-efficient direction. Everything processes locally through FFmpeg compiled to WebAssembly, so voice memos, ripped tracks, or downloaded audio never leave your browser tab.
Converting an MP4 video instead of a bare AAC file? If your AAC is wrapped in an M4A container rather than a raw stream, M4A to MP3 is the matching tool for that container.
