Make sure they are under 512kb, and under 30 seconds.
In your Profiles > Advanced > Active Profile > Phone > edit, make sure it is set to Tone or Vibrate and Tone.
You will get about 15 seconds of ring time on most devices. If you want longer, up to 30 seconds... Increase amount of rings before voicemail - BlackBerryFAQ.com
Good luck, and welcome to the BBF.