Lossless when the MKV already has AAC
Many MKV files carry AAC sound: recordings from OBS Studio and other screen recorders, many web videos and a large share of films and series encoded for smaller files. When you convert MKV to AAC with Original audio selected, that track is copied out without being decoded. It is fast, and the sound is bit for bit the same. The input list tells you whether a copy is possible.
If the MKV has Dolby Digital, DTS, FLAC, Opus or Vorbis sound, the track is decoded and encoded to AAC at the bitrate you choose.
The .aac file in practice
The result is a raw AAC stream in ADTS form. It is the simplest way to store AAC and is often what audio tools, encoders and streaming software ask for when they need a separate sound track. It also plays on most phones and many car stereos. Two limits are worth knowing: tags and cover art are poorly supported, and some players show the wrong length or seek slowly. For a music library, choose MKV to M4A, which holds the same AAC sound in a proper container.
Typical uses
- Pulling the game sound or commentary out of an OBS recording to edit it in another program.
- Getting a separate audio track to combine with a different video later, for example with Add Audio.
- Saving the sound of a lecture or webinar in a compact format.
Bitrate when encoding
For film sound from Dolby or DTS, 160 to 192 kbps gives a clean stereo result. Speech is clear at 96 to 128 kbps. If the source is lossless FLAC from a concert, use 256 kbps to keep the detail. When the track is already AAC, copying is always better than encoding it again at a lower bitrate, unless the file size matters more than quality. If you need a format that even very old devices play, MKV to MP3 is the alternative. The file stays on your computer during the whole process.