Avidemux Forum

Was working fine in prev version, now I can't save to mkv, only to avi

Tried nighty version - same result

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This is correct, Windows Media Audio is unsupported in MKV container (actually, in no container in earnest, even if technically AVI accepts everything). I would recommend to re-encode both audio and video using up-to-date codecs (if your target device supports HEVC or VP9 for video, then one of these codecs, else H.264 for best compatibility; AAC for audio). WMV and WMA is a dead-end.