You don't need to make your own folders.
The proper file to put the mp3s is /system/media/audio/ - where you will find folders for /alarms , /notifications , and /ringtones
There are two things to keep in mind when doing this:
If you are using full-sized .mp3's , you will put considerable strain on the system when it comes time to load one. In my experience, two minutes of mp3 works fine for an alarm, but such a file takes a few seconds to load, and will not make for a suitable ringtone, unless you trim the file first.
You will have to re-do this every time you flash a new ROM.
An alternative to this -- is to have them in /sdcard/media/ --
But if you have an older device, having them in the root directory allows for better performance.