Open the calendar application and press menu to get to the settings. There should be reminder settings where you set the ringtone sound for your calendar notifications. I believe the calendar uses the notification tones instead of the ringtone list.
And for an easy way to get your custom ringtones in their proper lists, create the following folders on your sd card.
SDcard\media\audio\ringtones
SDcard\media\audio\notifications
SDcard\media\audio\alarms
Copy and paste your ringtones files to whichever folder you want them to show up in. Reboot or disconnect your phone from the computer depending on the means you used to create and move the files around. Then go into any of your ringtone or notification tone selection lists and the appropriate tones will automatically be there. No more messing around with the music player to assign ringtones.
There is an extra bonus in doing it this way. If you ever have to factory reset your phone, the phone will read those folders and right away add to sounds to your selection list and all you have to do is reassign the tones to each application you set up a custom notification tone.
Hope that helps.
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now im thinking it may be better to keep the default beat box ringtone that came with the phone or rather just continue using that. its much less disturbing for work and long trips lol.