Note: You do not need an SD card or music for this trick. Go to the "Sound Channel" and tap "Record And Edit A Sound". Do not touch anything on the DSi. After about a minute, the Mario Bros. theme will play. Also at the very end of the audio it will begin to play a random audio clip from your library (not your SD card).
On the DSi Sound Channel, record a sound and go to the box with four squares. On each of the squares, you will see either a parakeet, low harmony, trumpet, or electric fan. If you click on one of them, it will alter the recorded sound according to the square you picked. If you click on one of the squares after it is already in use, it will change to a new sound. For example, if you are using the electric fan sound, it will change to a tunnel sound.
Note: This trick requires at least one song file on the console that can be played in "Music". While in the "Music channel" on the Nintendo DSi, start a song, then instantly pause it. Look at the bottom screen's shoulder instruments. Scroll through until the "No sound" icon is displayed on them. Press Up repeatedly until the screen with Mario running appears. Press "Play" and use L and R to make Mario jump. See how many coins you can collect within your favorite songs. Note: The shoulder button sounds need to set to "None" so that the audio does not become distorted, however you can still play with them if desired.