This effect is sort of hidden, but now I reveal it. It's really weird.

1. Open DSi Sound and record a sound (preferably your voice).

2. Go to Edit.

3. Play it backwards and put tunnel effect on it (go to the 4 squares and tap Electric Fan twice).

4. Overwrite it.

5. When finished it will automatically play backwards and will make a really weird cool effect.

TIP: You can overwrite it again so it will play the right way.

Enjoy!