Instructions
Tap Menu on the DSi home screen, touch Sound, and then press ̶0;Record and edit a sound.̶1;
Touch the recording that you want to add to. Each balloon in this menu is a recording and the half-filled balloons have extra space. The extra space can be used for additional recording.
Press the ̶0;Play̶1; button to play the sound and then touch and drag the playback position to the very end of the recording. Tap the ̶0;Overwrite̶1; button to open the recording menu.
Move the playback position to the far right again to prevent overwriting what you already have recorded. Press ̶0;Record̶1; to add to your recording.
Press ̶0;Stop̶1; when ready to save your modified recording.