REGEDIT4
[HKEY_CLASSES_ROOT\.mp3]
@="mp3file"

[HKEY_CLASSES_ROOT\mp3file\shell\Audacity]
@="オダシティで開く"

[HKEY_CLASSES_ROOT\mp3file\shell\Audacity\command]
@="\"C:\\Program Files\\Audacity\\audacity.exe\" %1"