If you got your phone unlocked / jailbroken using the ChevronWP7 tool, its time to check the next app which allows you to install new ring tone / change existing Ringtone / add ringtones in Windows Phone 7. You can select up to 5 sound files in WMA Format which can be set as ringtone when its gets deployed as package in your WP7 handset.
However the installation is like straight forward installation. This Custom Ringtone Manager creates a XAP file only which you will have to deploy using the Windows Phone 7 Developer SDK. By default it is not possible to add new ringtones to Windows Phone 7 but hopefully in next update we might see this feature active.
Grab the Windows Phone 7 Ringtone Manager from here and make sure the WMA formats are at 48 KHz.