The easiest case: AAC already inside
Since around 2008, most FLV files have used H.264 video with AAC sound. Recordings of live streams are almost always H.264 with AAC as well, because that is what streaming software sends. That means the audio you want is very often AAC already. When you convert FLV to AAC with Original audio selected, the tool copies the sound track out of the container. No decoding, no encoding, no loss, and even a long recording is done in moments.
Older FLV files carry MP3, Nellymoser or Speex sound. These are converted to AAC at the bitrate you choose. The tool tells you which case applies before you start.
What a .aac file is
The result is a raw AAC stream, often called ADTS AAC. It has no container, so it is simple and widely understood by audio tools, streaming software, many car stereos and Android players. It has two weak points: it stores tags poorly, and some players cannot show its exact length or seek quickly in it. If you want the file in a music library, on an iPhone or with cover art, choose FLV to M4A. It is the same AAC sound in an MP4 container, and the copy is just as lossless.
Good uses
- Saving the audio of a streamed podcast, concert or radio show exactly as it was broadcast.
- Getting a clean AAC track to use in another tool that rebuilds a video or a stream.
- Archiving many recordings in a small format without any extra quality loss.
If you need to re-encode
For non-AAC sources, or if you want a smaller file, set a bitrate. 96 kbps is usually enough for speech, 128 to 192 kbps for music. Re-encoding AAC to AAC at a lower bitrate always loses some quality, so only do it when the size really matters.
If the tool reports that the FLV has no sound track, the file contains only video, which happens with some screen recordings. To keep the video too, use FLV to MP4, which can copy both streams without re-encoding. For a format that every device plays, FLV to MP3 is the alternative. All processing stays on your computer.