Make MP3 Ringtones.
Step 1: Create the ringtone
The first thing you need to do is select the audio you're going to use. This could be music or a voice recording. I started with a high-quality MP3 file. Most information I read recommended using a clip no longer than 30 seconds so you need to edit the portion of the audio that you want to use down to that length. I used Adobe Audition for this because I happened to have it but a simpler, free, online tool that will do the job is at Make Your Own Ringtones - Free Ringtone Maker ... (Note: This site says it will download to your phone but I was never able to get that function to work. I wasn't able to get it's AAC converter to work either. Use the "Download to Computer" option.) I saved my MP3 file at 96kbps which is a fairly low quality. This is generally acceptable since the speaker on the phone isn't very good. You could use a higher quality but it will result in a larger file that consumes more storage space on the phone.
Step 2: Convert MP3 to proper format
You now need to convert the MP3 file to an M4A file. I used iTunes to do this since it's free and I already had it. In order for this to work, iTunes needs to be configured to import ripped CDs as AAC files. To do this, go to Edit > Preferences > Advanced and then click on the Importing tab. Set "Import Using:" to AAC Encoder. I used a custom value for the quality setting and again went with 96kbps. Click OK to update your settings. Once this is done, import your MP3 into iTunes and then right-click on it and pick "Convert Selection to AAC" from the pop-up menu. iTunes will create a new version of the file with an .m4a extension. You can locate it easily by right-clicking on the new entry and picking "Show In Windows Explorer."
Step 3: Send the M4A file to your phone
Now that you have an actual ringtone in a format phone will play, all that remains is to send the ringtone to your phone. There are many sites on the Internet that provide this service, but the easiest and most reliable I found was Free Ringtones, Logos, Mobile Videos - FunForMobile ... this site has free uploaders for various file types and requires no registration prior to using their tools. You'll of course use the free "Upload Ringtone" function found in the menu on the left side of their home page. You'll set your carrier to "Sprint," enter your mobile phone#, browse to the location where your M4A file is stored and then enter a name for the ringtone that will be used once it's on the phone. Then click "Upload."
After a short delay you should receive a text message on your phone that contains a link to your new ringtone. Hit the "Option" button on the right and select 'Go" from the menu. This will start up the web browser and take you to a Fun For Mobile page. Move down to the "Download" link with the directional keys and then click OK. The phone will pop up an alert that this is a "non-trusted file" because it is on a non-Sprint website. Just hit the left button to say "Continue" and the phone will download the ringtone. Note that if you do not have a data plan for your phone, data charges will apply.
After the ringtone downloads you can listen to it and assign it as a ringer. It will be stored in your "My Content" under "Ringers" or whatever.
So there you have it, just about everything you need to know to create and install custom ringertones on your phone. There may be an easier way to do this, but I haven't found it. Once you've done this once or twice, this becomes a really quick and easy way to use your own music on your phone. I hope you've found this helpful.