What is the filename of "New Messages Sound"?
It's located in the Resources folder inside Mail.app. Assuming you've left Mail.app in its default installed location, the path is:
/Applications/Mail.app/Contents/Resources/New\ Mail.aiff
It's located in the Resources folder inside Mail.app. Assuming you've left Mail.app in its default installed location, the path is:
/Applications/Mail.app/Contents/Resources/New\ Mail.aiff