ConvertTo-mp3.ps1

<#
.SYNOPSIS
Primarily used to convert .m4a and .mp4 audio files to .mp3

.PARAMETER Bitrate
Default bitrate is 128..

.PARAMETER Force
Overwrite existing .mp3 files without prompting for confirmation.

.PARAMETER FullQuality
Preserve full quality. Default is to use lame compression to save disk space.

.PARAMETER Info
Show ffmpeg info. Default is to run quietly.

.PARAMETER Input
Path to an .m4a file or a directory containing .m4a files. 
This is not a recursive operation.

.PARAMETER OutputPath
Path of output directory. Default is same directory as .m4a file.

.PARAMETER Recurse
Recurse directory tree converting all .m4a files, not recommended with -OutputPath

.PARAMETER Extension
File type extension to search for, defaults to .m4a
#>

# CmdletBinding adds -Verbose functionality, SupportsShouldProcess adds -WhatIf
[CmdletBinding(SupportsShouldProcess = $true)]

param(
	[Parameter(Mandatory = $true, Position = 0)] [string] $InputPath,
	[Parameter(Position = 1)] [string] $OutputPath,
	[int] $Bitrate = 128,
	[string] $Extension = '.m4a',
	[switch] $FullQuality,
	[switch] $Info,
	[switch] $Recurse,
	[switch] $Clean,
	[switch] $Force
)

Begin
{
	function EnsureConverter
	{
		if ((Get-Command ffmpeg -ErrorAction:SilentlyContinue) -eq $null)
		{
			Write-Host '... installing ffmpeg'

			if (!$WhatIfPreference)
			{
				choco install -y ffmpeg
			}
		}
	}


	function Convert
	{
		param($name)

		$mp3 = [IO.Path]::ChangeExtension($name, 'mp3')

		if ($OutputPath)
		{
			$mp3 = Join-Path $OutputPath (Split-Path $mp3 -Leaf)
		}

		$loglevel = 'quiet'
		if ($Info) { $loglevel = 'info' }

		$over = ''
		if ($Force) { $over = '-y' }

		if ($FullQuality)
		{
			Write-Host "... converting $name with full quality" -ForegroundColor Cyan

			if (!$WhatIfPreference)
			{
				ffmpeg -i $name -q:a 0 -map a -vn -loglevel $loglevel $over $mp3
			}
		}
		else
		{
			Write-Host "... converting $name" -ForegroundColor Cyan

			if (!$WhatIfPreference)
			{
				ffmpeg -i $name -b:a $Bitrate`K -vn -loglevel $loglevel $over $mp3
				# or...
				#ffmpeg -i $name -acodec libmp3lame -aq 2 -loglevel $loglevel $over $mp3
			}
		}

		# some commands like Test-Path don't like '[' in a path so those chars must be escaped
		$mp3 = [WildcardPattern]::Escape($mp3)
		if ($Clean -and (Test-Path $mp3))
		{
			$name = [WildcardPattern]::Escape($name)
			$item4 = Get-Item $name
			$item3 = Get-Item $mp3
			if (($item3.Length -gt 0) -and ($item3.LastWriteTime -gt $item4.LastWriteTime))
			{
				Remove-Item $name -Force -Confirm:$False
			}
		}
	}
}
Process
{
	if (!(Test-Path $InputPath))
	{
		Write-Host "... $InputPath is not found" -ForegroundColor Red
		return
	}

	EnsureConverter

	if ($OutputPath -and !(Test-Path $OutputPath))
	{
		Write-Host "... creating $OutputPath" -ForegroundColor Cyan
		New-Item $OutputPath -ItemType Directory -Force -Confirm:$False | Out-Null
	}

	if ($Extension -and ($Extension[0] -ne '.'))
	{ 
		$Extension = ".$Extension"
	}

	if ($Recurse)
	{
		(Get-ChildItem .\ -Filter "*$Extension" -Recurse).DirectoryName | select -Unique | % `
		{
			Push-Location ([WildcardPattern]::Escape($_))
			ConvertTo-MP3 .\ -Clean:$Clean -Force:$Force
			Pop-Location
		}
	}
	elseif ((Get-Item $InputPath) -is [IO.DirectoryInfo])
	{
		Get-ChildItem $InputPath -Filter "*$Extension" | % { Convert $_.FullName }
	}
	else
	{
		Convert $InputPath
	}
}