did u go thru this path?
C:\Program Files\Microsoft Games\Microsoft Flight Simulator X\Captain_Sim\757
there is sound configuration file that will open with notepad.
u can also try disabling (turning off) the 'UAC control' found under 'user accounts' in 'control panel' when u click on 'start'. then try changing the config file. u may need to restart the comp after turning the 'user acount control-uac' off.