extrax.ps1

Param($file)
$file = Get-Item $file
mkvinfo $file.FullName `
| &{
	Begin {
		$current = $NULL
	}
	Process {
		if (!$current) {
		}
		elseif ($_ -match '^\|  \+ ([^:]+): (.*)$') {
			switch -Exact ($Matches[1]) {
				'Language (IETF BCP 47)' {
					$current['language'] = $Matches[2]
				}
				'Name' {
					$current['name'] = $Matches[2]
				}
				'Track type' {
					$current['type'] = $Matches[2]
				}
				'Track number' {
					$current['track'] = ($Matches[2] -replace '[^0-9].*','') - 1
				}
				'Codec ID' {
					$current['extension'] = switch -Exact ($Matches[2] -replace '[^A-Z0-9_].*', '') {
						'A_AAC'   { '.aac' }
						'V_MPEG4' { '.m4v' }
						'S_TEXT'  { '.srt' }
					}
				}
			}
		}
		else {
			$current['filename'] += '_' + ($current['track'] + 1)
			if ($current['name']) {
				$current['filename'] += '_' + $current['name']
			}
			if ($current['type'] -in 'audio','subtitles') {
				$current['filename'] += (" ($($current['language']))","_$($current['language'])")[!$current['name']]
			}
			$current['filename'] += $current['extension']
			$current
			$current = $NULL
		}
		
		if ($_ -match '^\| \+ Track') {
			$current = @{
				filename = $file.BaseName
			}
		}
	}
} `
| &{
	Begin {
		$commands = $file.FullName,'tracks'
	}
	Process {
		$commands += $_['track'],$_['filename'] -join ':'
	}
	End {
		mkvextract @commands
	}
}

vtt.ps1

gi *.srt `
| %{
	$item = $_
	&{
		'WEBVTT'
		"Language: $($item.BaseName -replace '^.*_','' -replace '^.* *\((.*)\)$','$1')"
		''
		gc $item
	} `
	| %{
		if ($_ -match '^((^|:)\d{2}){2,3},\d{3}(( --> |:)\d{2}){2,3},\d{3}(\s|$)') {
			([Regex]',').Replace($_, '.', 2)
		}
		else {
			$_
		}
		"`r`n"
	} `
	| Out-File -NoNewline -LiteralPath ($_.BaseName + '.vtt')
}

webmp4.sh

ffmpeg \
    -i "$1.mp4" \
    -c:v libx264 \
    -crf 23 \
    -c:a aac \
    -movflags faststart \
    -maxrate 2M \
    -bufsize 2M \
    "$dest/$1.mp4"

curl \
    --form subrip_file=@"$1.srt" \
    -o "$dest/$1.vtt" \
    'https://atelier.u-sub.net/srt2vtt/'

gi *.srt | %{ curl --form "subrip_file=@$($_.Name)" -o ($_.Name -replace 'srt$','vtt') 'https://atelier.u-sub.net/srt2vtt/' }
MP4Box -add x_1.m4v -new x.mp4; MP4Box -add x_2.acc x.mp4