Conver to mp3
If you do not have a webserver but need to convert and download your favorite mp3 files to ringtones you can take a look to the BlackBerry Ringtone Generator. By selecting the fragment of your mp3 file you can take influence on the size of the resulting mp3 ringtone file.
Another good PC-based audio editing software is Audacity. It can be found free by using Google.
For an great on-line editing/conversion application, try MakeOwnRingtone. Another good online conversion site is www.cellsea.com

Note: it is recommended to have not file sizes exceeding 512 kB for some models. Most newer BlackBerry devices can handle bigger files but a ringtone below 512 kB will always work.
Similarly, there is no need to have a ringtone longer than 30 seconds, as the various carrier's do not allow a ring time before voicemail longer than 30 seconds.

Medthod 2 Please note that this conversion will only work on Newer Generation Devices (7100, 7290, 8700, 8100)

• You'll need a couple things to make an .adp ringtone that'll play on the New Gen BlackBerry
  • an audio editor (Cool Edit, etc)
  • a webserver
  • The Philips version of Mobile Phone Tools. Get it here:

http://www.p4c.philips.com/files/c/ct5398_atusavp2/ct5398_atusavp2_ics_eng.zip
First edit your .wav or .mp3 down to about 15 seconds. We need to make sure that the final .adp file is under 120K, which seems to be the max file size the 7100 can handle. Save the file as an MS PCM wav.
Next, install Mobile Phone Tools. Skip the part that asks for your Identifier, fax #, etc. Also, whenever you start the program it's gonna ask how your phone is connected to your PC. Since it only works with Philips phones, click cancel.
Mobile Phone Tools can apparently be used to sync contacts, email, etc, but we have a Blackberry so we don't care about that functionality. All we need this program for is the Multimedia Center. Click the Menu button, then Tools, then Multimedia Center. Once this opens click on the button that says Melody Studio. Open your wav, click Save As, and select Oki ADPCM as the file type.
Now upload this file to your web server. Before you can download to the 7100, you need to make sure that the correct MIME type has been set. MIME types tell a browser how to handle certain file extensions. You want to add the MIME type "audio/adpcm adp". The way to do this differs from server to server, so check your web server's documentation or speak w/ your hosting company.
Once you've got the MIME type set, and the .adp file uploaded, open up the BB browser and navigate to the URL where your .adp is. It can take a few minutes for it to load, so don't panic. Once the Media Player has loaded and your ring tone is playing, click the scroll wheel and choose save. It'll ask for confirmation that you want to save it (you can also change the file name at this point).
That's it. You've now got a ringtone infinitely better than midi on your 7100 ;)
I'll be updating this as more information is known. There's still a few bugs to work out as far as the absolute max file size, etc.
(Credit Goes to HiRoll3r from BlackBerryForums.com)