script.ps1

# Get the collection of files.
$files = Get-ChildItem -Path '.' -Filter '*.opus'
$total = $files.Count

# No need for the synchronized hashtable as it was unused.

Write-Progress -Activity "opus -> m4a + fixing date..." -Status "Starting..." -PercentComplete 0

# Counter for processed items.
$processedCount = 0

# ForEach-Object -Parallel returns output to the pipeline. We can process this output.
$files | ForEach-Object -ThrottleLimit 8 -Parallel {
    # It's good practice to wrap external command calls in a try/catch
    # block to handle potential errors gracefully within a parallel loop.
    try {
        $file = $_

        # Capture the output of ffprobe.
        # Directly casting to [string] is simpler than Out-String.
        $original_date = [string](ffprobe -v quiet -of default=noprint_wrappers=1:nokey=1 -show_entries stream_tags=date "$file")

        # Modify the date format if it exists.
        if ($original_date) {
            # Trim any whitespace or newlines.
            $original_date = $original_date.Trim()
            # This assumes the date format is YYYYMMDD.
            $modified_date = ($original_date.Insert(4, '-')).Insert(7, '-')
        } else {
            $modified_date = ""
        }

        $target = $file.BaseName + ".m4a"

        # Run ffmpeg.
        ffmpeg -v quiet -i "$file" -c:a aac -map_metadata 0:s:a:0 -metadata "date=$modified_date" "$target" -y

        # Output a value to the pipeline to signal completion.
        # Returning a boolean $true is slightly more efficient than a full string.
        return $true
    }
    catch {
        # Optionally, write errors to the console from the parallel thread.
        Write-Error "Failed to process '$($file.Name)': $_"
    }

} | ForEach-Object {
    # This block runs sequentially in the main session.

    # Increment the counter for each item received from the parallel block.
    $processedCount++
    
    $percent = (($processedCount / $total) * 100)
    
    Write-Progress -Activity "opus -> m4a + fixing date..." -Status "Processed $processedCount of $total files" -PercentComplete $percent
}

# Final status update.
Write-Progress -Activity "opus -> m4a + fixing date..." -Status "Done!" -Completed

Experimental!