I do not think on the 8310 you can save voice notes. I read that you can on some devices though the 8310 is not one of them. I guess the best way is to mms/email it to yourself (phone)and then save it from there. Once that is done just set it as a ringtone either in the media player menu save or in your profile settings....
Yes, you can do it. Here are the steps I found to do it:
record your voice note and rename it if you would like. Then press the stop icon (the square). This brings you to your "view voice notes" screen. Select the one you want (assuming there are more than one in your list), Press the menu button and select "set as ring tone". I haven't actually tried it yet but it looks as if this might answer your question.