You can go to the market and download "Audio Manager"
once you have that, go to "SYSTEM VOLUME" take it to "0"
Should work...worked on mine with D1-MIUI-0.11.26
Good Luck!!
Try this:
Go into settings
>under common tab you will see MMS Settings
>under storage settings you will see MMS limit
>click on it and you should be able to set it
GOOD LUCK...