A PS script which uses ffmpeg to convert all audio files of a specified extension in a directory to another codec.

ConvertAudio.ps1

class Files {
	hidden [String] $iExt
	hidden [String] $oCodec
	hidden [String] $oExt
	hidden [String] $iDir
	hidden [String] $oDir
	hidden [System.Object] $files

	Files ([String] $iExt, [String] $oCodec, [String] $oExt) {
		$this.iExt = $iExt
        $this.oCodec = $oCodec
		$this.oExt = $oExt
		$this.iDir = $PSScriptRoot
		$this.oDir = "$($this.iDir)\$oCodec"
        $this.files = Get-ChildItem -Path "$($this.iDir)\*" -Include "*.$($this.iExt)"
	}

	Files ([String] $iExt, [String] $oCodec, [String] $oExt, [String] $iDir) {
		$this.iExt = $iExt
        $this.oCodec = $oCodec
		$this.oExt = $oExt
		$this.iDir = $iDir
		$this.oDir = "$($this.iDir)\$oCodec"
        $this.files = Get-ChildItem -Path "$($this.iDir)\*" -Include "*.$($this.iExt)"
	}

	Files ([String] $iExt, [String] $oCodec, [String] $oExt, [String] $iDir, [String] $oDir) {
		$this.iExt = $iExt
        $this.oCodec = $oCodec
		$this.oExt = $oExt
		$this.iDir = $iDir
		$this.oDir = $oDir
        $this.files = Get-ChildItem -Path "$($this.iDir)\*" -Include "*.$($this.iExt)"
	}
    
    [String] GetInExt() {
		return $this.iExt
	}

    [String] GetOutCodec() {
		return $this.oCodec
	}

    [String] GetOutExt() {
		return $this.oExt
	}

    [String] GetInDir() {
		return $this.iDir
	}

    [String] GetOutDir() {
		return $this.oDir
	}

	[System.Object] Get() {
		return $this.files
	}

	[Void] Convert() {
		New-Item -ItemType Directory -Force -Path $this.oDir | Out-Null

		foreach ($file in $this.files) {
			Write-Host "`nConverting file " -NoNewline
			Write-Host ($file.Name) -NoNewline -ForegroundColor Cyan
			Write-Host " to $($this.oCodec)."

			ffmpeg -hide_banner -loglevel warning -i $file.FullName -c:a $this.oCodec "$($this.oDir)\$($file.BaseName).$($this.oExt)"
		}
	}
}

function Prompt-Properties([Files] [ref] $files){
	$title =  "Audio Conversion"
	$message = "`nDo you want to perform a conversion with the following settings?"
	$message += "`n    Input directory path: $($files.GetInDir())"
	$message += "`n    Input file extension: $($files.GetInExt())"
	$message += "`n    Output codec: $($files.GetOutCodec())"
	$message += "`n    Output directory path: $($files.GetOutDir())"
	$message += "`n    Output file extension: $($files.GetOutExt())"

	$yes = New-Object System.Management.Automation.Host.ChoiceDescription "&Yes", `
		"Converts all files with file extension $($files.GetInExt()) in directory $($files.GetInDir()) to file extension $($files.GetOutExt()) and audio codec $($files.GetOutCodec()) and outputs them to directory $($files.GetOutDir())."

	$set = New-Object System.Management.Automation.Host.ChoiceDescription "&Set", `
		"Prompts for each setting to be set (input directory path, input file extension, output codec, output directory path, and output file extension)."

	$no = New-Object System.Management.Automation.Host.ChoiceDescription "&No", `
		"Does not convert any files and exits the script."

	$options = [System.Management.Automation.Host.ChoiceDescription[]]($yes, $set, $no)

	$result = $host.ui.PromptForChoice($title, $message, $options, 0)

	switch ($result) {
		0 {
			return $files
		}

		1 {
			Write-Host "`nIf a value is not set, the default will be used."

			$iDir = Read-Host -Prompt "`nInput directory path"
			$iExt = Read-Host -Prompt "Input file extension"
			$oCodec = Read-Host -Prompt "Output audio codec"
			$oDir = Read-Host -Prompt "Output directory path"
			$oExt = Read-Host -Prompt "Output file extension"

			if ($iExt -eq '') {
				$iExt = $files.GetInExt()
			}

			if ($oCodec -eq '') {
				$oCodec = $files.GetOutCodec()
			}

			if ($oExt -eq '') {
				$oExt = $files.GetOutExt()
			}

			if ($iDir -eq '' -and !($oDir -eq '')) {
				return [Files]::new($iExt, $oCodec, $oExt, $files.GetInDir(), $oDir)
			}

			if (!($iDir -eq '') -and $oDir -eq '') {
				return [Files]::new($iExt, $oCodec, $oExt, $iDir)
			}

			if (!($iDir -eq '') -and !($oDir -eq '')) {
				return [Files]::new($iExt, $oCodec, $oExt, $iDir, $oDir)
			}

			return [Files]::new($iExt, $oCodec, $oExt)
		}

		2 {
			exit
		}
	}
}

function Prompt-Convert([Files] [ref] $files) {
	$title =  "Found $($files.Get().Count) $($files.GetInExt()) files eligible for conversion:"

	foreach ($file in $files.Get()) {
	   $title += "`n    $($file.Name)"
	}

	$message = "`nDo you want to convert the files above to $($files.GetOutCodec())?"

	$yes = New-Object System.Management.Automation.Host.ChoiceDescription "&Yes", `
		"Converts the files listed above to file extension $($files.GetOutExt()) and audio codec $($files.GetOutCodec())."

	$no = New-Object System.Management.Automation.Host.ChoiceDescription "&No", `
		"Retains the current audio codec and file extension for each file listed above."

	$options = [System.Management.Automation.Host.ChoiceDescription[]]($yes, $no)

	$result = $host.ui.PromptForChoice($title, $message, $options, 1)

	switch ($result) {
		0 {
			$files.Convert()
			Read-Host -Prompt "`nFinished converting $($files.Get().Count) files. Press [Enter] to exit"
		}

		1 {
			Read-Host -Prompt "`nNo files were converted. Press [Enter] to exit"
		}
	}
}

[Files] $filesDefault = [Files]::new('wav', 'flac', 'flac')
[Files] $filesConvert = Prompt-Properties ([ref] $filesDefault)
Prompt-Convert ([ref] $filesConvert)