To make WMP11 x64 Edition your Default Player;
1. Open "Command Prompt" and "Run as Administrator" and type the following command:
%windir%\system32\unregmp2.exe /SwapTo:64
2. Open the Registry Editor (regedit) and navigate to “HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\App Paths\wmplayer.exe”.
3. Double-click the registry subkey “Path” and change to “%Programfiles(x86)%\Windows Media Player” (without quotes).
4. Double-click the registry subkey “(Default)” (normally the first entry) and change that to “%ProgramFiles%\Windows Media Player\wmplayer.exe” (with quotes).
To change back to Windows Media Player 11 x86 32-bit edition, just reverse your steps or restore the back-up that you made before the changes.