Convert MKV to MP3 - extract audio
Pull the audio out of an MKV as an MP3 file. Wave decodes only the audio track - the video is discarded - and re-encodes to 192 kbps MP3 with LAME. Everything happens locally. If it's unclear how "container" and "codec" separate here, Codecs vs containers covers the wrapper-vs-content framing.
What actually changes
MKV audio tracks are usually AAC, AC-3, DTS, or FLAC. MP3 is a different codec entirely, so extraction is always a re-encode. Wave uses libmp3lame - the reference-quality open-source MP3 encoder - at 192 kbps CBR joint-stereo.
Because Wave never has to touch the video track, the extract is fast - often 10x faster than a full MKV-to-MP4 video conversion. An hour of typical MKV audio finishes in well under a minute on a modern laptop.
MKV was designed to hold rich audio configurations: multiple tracks (director commentary, alternate dubs, multiple language options), lossless formats like FLAC and DTS-HD Master Audio, and multi-channel layouts up to 7.1 surround. Wave's extract handles the codec diversity - the on-device ffmpeg engine decodes AAC, AC-3, E-AC-3, DTS, DTS-HD, FLAC, Vorbis, Opus, TrueHD, and PCM without additional configuration. Whatever the source track is, it decodes correctly and re-encodes to MP3 through LAME.
The multi-track situation is the one limitation. If your MKV carries three audio tracks (English, Japanese, commentary), Wave extracts the default-flagged one. Reading a specific alternate track requires a desktop ffmpeg with '-map 0:a:1' or similar. This is on the roadmap for a track-picker UI in a future release.
Three steps.
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01
Step 1
Drop the MKV file onto the card.
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02
Step 2
Pick MP3 as the output format.
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Step 3
Convert and save the MP3.
Answers, specifically.
Which audio track gets picked if the MKV has several?
The default track, which the source file marks. If the MKV has no default flag, Wave picks the first audio stream.
Will AC-3 or DTS lose quality?
They're surround-encoded - Wave downmixes to stereo before re-encoding to MP3. The MP3 sounds like a good stereo mix, not a surround field.
Can I extract as FLAC or WAV instead?
No - MP3 is the only audio output Wave produces. If you need a lossless working file, run the MP3 through a dedicated audio tool afterwards. Wave stays focused on video → MP4 with MP3 as the single audio shortcut.
Can I extract only a segment?
Not yet. Wave extracts the entire audio track; trimming lands in a later release.
How big will the MP3 be?
192 kbps works out to roughly 1.4 MB per minute - an hour of MP3 is about 85 MB.
What if the source audio is FLAC (lossless)?
FLAC decodes to raw PCM and then gets re-encoded through LAME to MP3. There is a small quality drop from lossless to MP3, but at 192 kbps CBR the difference is inaudible on almost any consumer playback gear. If you need a lossless output, use a desktop tool to extract the FLAC track directly.
The MKV audio is Opus. Does that convert cleanly?
Yes. Opus decodes correctly through ffmpeg-wasm and re-encodes to MP3. This is a common configuration for MKVs made from WebM sources (YouTube downloads muxed into MKV, for instance) - the extract works exactly the same as for AAC or AC-3.
Are audio delays or offsets preserved?
The starting timestamp is preserved. If the MKV's audio track has a fixed delay relative to video (some scene releases add a 25-40 ms delay for A/V sync), the MP3 starts at that offset - which is invisible for audio-only playback but visible if you later re-mux with a new video.