Script to normalize MP3 and FLAC: Uses ffmpeg-normalize and ffmpeg

ffmpeg-normalize-all.ps1

# Normalize mp3 and flac files in current directory using ffmpeg-normalize

# scoop install ffmpeg
$ffmpeg = 'ffmpeg.exe'
# scoop install python
# pip3.exe install ffmpeg-normalize
$ffmpeg_normalize = 'ffmpeg-normalize.exe'

function Get-MeanVolume {
    Param([Parameter(Mandatory = $true)][string]$File)

    $psi = New-Object Diagnostics.ProcessStartInfo
    $psi.FileName = $ffmpeg
    $psi.RedirectStandardError = $true
    $psi.RedirectStandardOutput = $false
    $psi.UseShellExecute = $false
    $psi.Arguments = "-hide_banner -nostdin -i `"{0}`" -filter:a volumedetect -f null /dev/null" -f $File
    $proc = New-Object System.Diagnostics.Process
    $proc.StartInfo = $psi
    $proc.Start() | Out-Null
    # Important: ReadToEnd must come before WaitForExit!
    $err = $proc.StandardError.ReadToEnd()
    $proc.WaitForExit() | Out-Null

    [double]($err -split "`r`n" | Select-String 'mean_volume: (.+) dB').Matches.Groups[1].Value
}

function Normalize-File {
    Param(
        [Parameter(Mandatory = $true)][string]$File,
        [int]$TargetLevel = -18,
        [string]$OutputFolder = 'NORMALIZED'
    )

    # Extension without dot.
    $ext = [IO.Path]::GetExtension($File).Substring(1)

    # Why -c:a ext ?
    #  See https://github.com/slhck/ffmpeg-normalize#could-not-write-header-for-output-file-error
    #  The conversion does not work and I get a cryptic ffmpeg error!

    & $ffmpeg_normalize --output-folder $OutputFolder --normalization-type rms --target-level $TargetLevel --force -c:a $ext --extension $ext $File
}

if (-not (Get-Command $ffmpeg -CommandType Application -ErrorAction SilentlyContinue)) {
    "'{0}' not found." -f $ffmpeg
    return
}

if (-not (Get-Command $ffmpeg_normalize -CommandType Application -ErrorAction SilentlyContinue)) {
    "'{0}' not found." -f $ffmpeg_normalize
    return
}

Get-ChildItem -File | Where-Object Extension -in '.mp3', '.flac' | ForEach-Object {
    # --target-level -13 (Average RMS Power in dbFS) is what pynormalize uses https://pypi.org/project/pynormalize/
    # but that seems too aggressive, so I'm using -18.
    [int]$targetLevel = -18

    $_.Name

    [double]$level = Get-MeanVolume -File $_.FullName

    if ([int]$level -ge [int]$targetLevel) {
        "`tLevel $level is fine. Skipping"
    }
    else {
        Normalize-File -File $_.FullName -TargetLevel $targetLevel -OutputFolder 'NORMALIZED'
    }
}