Convert MTS (and M2TS) to MP3 - extract audio
AVCHD camcorder files carry AC-3 or LPCM audio alongside their H.264 video. Wave discards the video track, decodes the audio, and re-encodes it to a 192 kbps MP3 you can share or archive.
What actually changes
MTS and M2TS are the AVCHD container used by Sony and Panasonic consumer camcorders and AVCHD Blu-ray discs. Audio inside is nearly always AC-3 (Dolby Digital, stereo or 5.1), with some older Panasonic camcorders and AVCHD Blu-ray tracks using LPCM instead. Both are LGPL-clean codecs and Wave decodes them locally with no server involved.
Because the video track is discarded entirely, extract runs several times faster than a full MTS-to-MP4 conversion - an hour-long camcorder recording usually finishes in under a minute. Wave uses LAME (libmp3lame) at VBR around 190 kbps, which is transparent for the natural-sound recordings camcorders typically produce.
Three steps.
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Step 1
Drop your .mts or .m2ts file onto the card.
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Step 2
Pick MP3 as the output format.
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Step 3
Convert and save the MP3.
Answers, specifically.
5.1 surround from an AVCHD Blu-ray - how is it handled?
Wave downmixes to stereo before encoding to MP3. MP3 doesn't carry multi-channel audio at all, so a downmix is the only path; the result sounds like a good stereo mix, not a surround field.
Which audio track gets picked if there are several?
Wave uses the container's default-flagged track, which is what the camcorder or authoring tool marked as primary. Multi-track extraction is on the roadmap.
How fast is it?
An hour of MTS audio typically re-encodes in 30–45 seconds on a modern laptop - much faster than the full video conversion, because Wave never has to decode a single frame.
What if my source file is enormous?
Wave's per-file ceiling is 8 GB. Even a 30 GB Blu-ray-source .m2ts would exceed it - split it first with any AVCHD-aware tool, or drop the individual clip files rather than the concatenated version.
Are timecode markers preserved?
MP3 has no equivalent for AVCHD timecode, so those are dropped. Basic ID3 tags (title from the filename) are written where available.