I've had my N900 flashed after the device drive corrupted. However I dont have any of the standard ringtones and sounds. Everything under the folder Nokia N900 (which I believe is the MyDocs folder) is missing.
Be aware that if you reflash the MMC, you'll lose all of your conversations, files and contacts. Make sure you transfer anything valuable off the phone first.
Someone may correct me if I'm wrong, but I believe that any backups done on-device are also erased by reflashing the MMC, so you'll lose them too if you don't copy them off the device first.
Make sure the file is in the directory you're running flasher from, and maybe try renaming the file to something shorter - the long names can run into limitations in Windows.