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You're listening to something on YouTube, Apple Music, or your iPhone's voice memos. The audio codec is almost certainly AAC. It was designed to replace MP3, and by every technical metric it succeeded — better quality at equivalent bitrates, more efficient compression, wider adoption than most people realise.
AAC is usually wrapped in an M4A or MP4 container rather than standing alone as a bare .aac file, which is why most people encounter AAC without knowing it. The format is backed by a consortium including Dolby, Sony, and Nokia, and it's the mandatory audio codec for MPEG-4 video. This means every MP4 video you've ever watched almost certainly used AAC for its audio track.
Every modern device plays AAC — phones, browsers, media players, streaming boxes. The only reason MP3 persists is momentum and familiarity. For new audio encoding, AAC at 256 kbps is effectively transparent (indistinguishable from the source for most listeners). If you need to convert AAC to MP3 for an older device, fwip handles it locally.
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