I'm actually having this same problem, and it's 2 years later. I believe that I have found the solution though.
I'll start out by saying I got my Droid Pro about a week ago.
I downloaded the "Voice Recorder" app by Mamoru Tokashiki.
I would record audio for ringtones (generally song snippets because I'm too lazy, or stupid, or I like to make more work for myself than downloading the actual ringtones). I would retitled the files (.3gp) and hit "Set as Ringtone." Then if I toggled my volume rocker it would say Silent Ringtone Selected.
I came to these forums and immediately knew that I didn't want to root my phone or do anything like that. So a lot of people suggested it was the software. I ditched the software and downloaded "Tape-a-talk" by Markus Drosser. It records in both .3gp and .wav. I use the .wavs because they sound so much better.
It worked for me once, but then I started getting the same message. It couldn't have been a problem with the software because I ran two different apps. It's something with the way that the android system saves the files.
I tinkered around with a process that is working for me now (it involves a lot of turning on and off which is annoying, so get prepared).
1. Open recording app (I use tape-a-talk) and record your audio. It shouldn't matter the duration (I thought I was getting the silent ringtone error because my files were too long, but that's not the case).
2. Immediately set the recording (labeled with some generic date like 03-19-2012.wav) as your ringtone. Toggle the volume rocker to check if you get the silent ringtone selected. (If you do, then try re-recording as .3gp. If that still doesn't work then this process probably won't work because something else major is wrong with your phone).
3. Turn off the phone.
4. Power back up (30s downtime).
5. Open up Settings>Sounds>Phone Ringtone. If 03-19-2012 is set as your ringtone then you should be pretty okay. Go back to the home page and toggle the volume rocker again to double check.
6. Turn off the phone then power it back up.
7. Go back into your recording app and find the generic file, rename it to what you want.
8. Set the newly titled .wav or .3gp as your ringtone.
9. Toggle the rocker to make sure it saved properly.
10. If it did, then turn your phone off. Turn the phone back on and open Settings>Sounds>Phone Ringtone.
If everything worked it should have your ringtone set and saved with the new file name.
Then you're free to enjoy.
That process works for me.
Some people may still claim it is the software, but in my experience it is a matter of how your phone ends up saving the recorded files.
This information is useless if you're talking about downloading a ringtone, btw.
Hope this helps anyone in the future.