It turns out there’s a bit of a secret to getting custom ringtones to sync over to Windows Phone 7.5 Mango. The Windows Team Blog has just posted about it, and it’s certainly not intuitive, but at least it will work. What you have to do is make a small WMA/MP3 file that’s less than 1Mb, and less than 39 seconds long with no DRM. Then in Zune on the desktop, you have right-click the file, edit song info, and change the Genre field to Ringtone. Then when you sync the song to your Windows Phone, it will show up in the ringtones settings as a custom ringtone.
Third party developers will also be able to make apps that can add custom ringtones.
Source: Windows Team Blog