Open iTunes on your PC. Click the "Edit" button on the top toolbar then click "Preferences" on the drop-down menu. Alternatively, hold "CTRL" and "," at the same time.
Click "Import Settings" in the window that pops to bring up the "Import Settings" screen. Where it says "Import Using: AAC Encoder" click the drop-down box and change "AAC Encoder" to "WAV Encoder". Click "OK" then "OK" again to bring you back to the iTunes library screen.
Import the M4A file you wish to convert if it isn't already there. Right-click the name of the file you wish to convert in your iTunes library, then click "Create WAV Version" in the menu that pops up. iTunes automatically creates a WAV file version of the M4A you requested in the M4A's original parent folder. The WAV file automatically appears in your iTunes library and is available for immediate playback.