Bulk Volume Change

param (
    # The path to a folder with music files in it. Make sure you surround this in quotes if it contains spaces.
    [Parameter(Mandatory = $true)]
    [string] $Path,

    # A percentage to increase the volume by, e.g. 1.5 for a 50% increase.
    [Parameter(Mandatory = $true)]
    [double] $Volume
)

# Create a temporary directory if it doesn't already exist
$tempDir = (Join-Path -Path $Path -ChildPath 'temp')
if (-not (Test-Path -Path $tempDir)) {
    New-Item -ItemType Directory -Path $tempDir
}

foreach ($file in (Get-ChildItem -Path $Path -File)) {
    # Need to store the output somewhere else temporarily, since FFMPEG can't edit in-place
    $tempFile = (Join-Path -Path $tempDir -ChildPath $file.Name)

    Start-Process -FilePath 'ffmpeg' -Wait -NoNewWindow -ErrorAction Stop -ArgumentList @(
        '-y'    # Force overwrite
        '-i', "`"$file`""   # Input file
        '-filter:a', "volume=$Volume"   # Boost volume
        "`"$tempFile`"" # Output file
    )

    # Move temp file over input file
    Move-Item -Path $tempFile -Destination $file -Force
}