Instructions
Check the circular headphone jack on the side of the DSi to make sure there are no headphones plugged into the jack. If there are headphone plugged in, this is likely the reason the mic isn't functioning. Remove any headphones connected to the jack and the mic will be active.
Check the game to make sure it supports the microphone feature. This information can be found in the game manual or on the packaging.
Tap "Settings" in the main menu of the DSi to open the "Settings" window.
Tap the right arrow twice to proceed to the third page of "Settings."
Tap "Mic Test" from the list of options.
Speak directly into the microphone (located in the center of the connector hinge) and the volume bar on screen will light up when it detects sound. If it detects no sound, your mic has likely failed and you will need to have it replaced.