Download "Any Video Converter" freeware (see References).
Launch the program from your Start menu.
Click on the "Add Video" button at the navigation pane.
Browse to navigate to the location that has the WMV file you want to convert into an MP3. Single-click on the file and click "Open."
Click to drag the left and right trim buttons on your preview screen to choose the specific track you wish to convert into an MP3.
Click to choose "MP3 Audio (*.mp3)" as your Output type and click "Convert."