First please ensure that the inserted game supports the Nintendo DS Lite microphone feature.
Check whether this problem only occurs when playing a certain game or if it occurs with all games. If this only occurs with a particular game verify if it offers a menu option for adjusting the microphone. You can use this option to adjust the sensitivity level for the microphone. (You will find more information on settings options in the respective game manual).
If the malfunction occurs with all games, the Nintendo DS Lite will need to be returned to Nintendo Australia for inspection/repair.