Here is working code:

// wise 5.30.24 - webm stream duration hack
$sFile = $mixedFiles['tmp_name'][0];
$sTempFile = BX_DIRECTORY_PATH_ROOT.'tmp/'.basename($sFile).'.webm';

$command = BX_SYSTEM_FFMPEG . ' -i ' . $sFile . ' -vcodec copy -acodec copy '.$sTempFile.' 2>&1';
$output = shell_exec($command);

$iDuration = BxDolTranscoderVideo::getDuration($sTempFile);
@unlink($sTempFile);

Works like a charm. Figured I'd post my code so anyone else needing this can benefit.

😎👍