# Version 1
Get-ChildItem *.wma | ForEach-Object {
    ffmpeg -i $_.Name -acodec libmp3lame -ab 192k ($_.BaseName + ".mp3")
}

# Version 2 - one liner
gci *.wma | % { ffmpeg -i $_.Name -acodec libmp3lame -ab 192k ($_.BaseName + ".mp3") }