iTunesToOpus.ps1

$itDir = "D:\iTunes Media\Music\"
$dstDir = "D:\Music\"
$ffmpeg = "C:\Users\greg\Downloads\ffmpeg-20170321-db7a05d-win64-shared\bin\ffmpeg.exe"

$itLib = Get-ChildItem -Include @("*.m4a", "*.aac", "*.mp3", "*.flac") -LiteralPath $itDir -Recurse
foreach ($path in $itLib) {
    if ((Get-Item -LiteralPath $path.FullName).Attributes -band [IO.FileAttributes]::Directory) {
        Write-Output "Directory $($path.FullName)"
    } else {
        $destPath = ([io.path]::ChangeExtension($path.FullName, "opus")).Replace($itDir, $dstDir)
        $destDirPath = (($destPath -split '\\')[0..(($destPath -split '\\').count -2)] -join '\')
        Write-Output "File $($path.FullName) → $destPath"
        if (-not (Test-Path -LiteralPath $destDirPath)) {
            md -Force $destDirPath
        }
        if (-not (Test-Path -LiteralPath $destPath)) {
            & $ffmpeg -i $path.FullName -c:a libopus -b:a 112k -metadata account_id="" -metadata compatible_brands="" -metadata rating="" -metadata comment="" -metadata creation_time="" $destPath
        }
    }
}