You may need to edit the sound files. Some sound effects and pieces of music will be the right length.
But letís say, for example, on one sound effects CD thereís a great drum roll thatís almost one minute long. If you want it shorter, youíll have to cut it down. Copy the sound file off the CD, but turn it into a WAV file, not an MP3. Then load it into software that will let you edit the length of the file.
The best free option currently available is Audacity. Itís open-source and does everything you need.
Learn how to load a WAV file into it and how to edit the sounds so you have what you need. You can ignore the other features, unless you want to mess with the sounds and do things like make them sound echo-y.
Once youíve edited it to the right length, save the file as an mp3 using Audacity.