Generally, the BlackBerry will not allow you to install a sound file into Device Memory. It does this because your file free is needed for storage and execution of apps. Sound files would jam your Device Memory very quickly. A media card can be quite inexpensive these days and resolve your dilemma quickly.
Posted from my CrackBerry at wapforums.crackberry.com
I'm far from a BB wiz, but if you really don't want to/can't get a mini chip, you can create the ringtone yourself on the pc, then import/email it to yourself. I think your best bet is making one at a free creator site (or choosing one) like ventones, phonezoo, etc., then emailing it to yourself. The email will show up on your phone and the BB will recognize the file as a RT and you can then save it. You can also get tons of RT here. I don't usually use my actual media for my RT once its on my BB because the BB won't let me pick the part of the song I want - it's strictly the 1st 20-30s. The sites I mentioned actually let you pick the part of the song you want as a RT. Hopes this helps!
You attach the MP3 file you created at ventones to an email message and send it to an email address set up on your Berry. It will give you the option to save the MP3 file. You then save it to the BlackBerry\Ringtones folder.