If the sound of your voice is difficult to hear and comes through sounding low or far away, turn up the volume of the cellular phone that is connected to the Bluetooth audio system and speak clearly while facing the system.
If the low sound continues once you have raised the cell phone volume, if applicable to your system, check the Mic Gain adjustment. This allows you to switch between two volume levels to set a suitable level for the other party during a phone call.
If the microphone is properly located, and there is still no improvement after increasing the volume, check whether there is a connection error. For more on connecting and installing the microphone, refer to the instruction manual for each device.