Speaker is in important device of any mobile. Through speaker sound will come. Mobile phone may have speaker in different regions of the mobile.
3.5 mm jack
3.5 mm jack is an important component of a mobile. A headphone can be connected with the mobile through the 3.5 mm jack. It can also be acted as an output device through this jack. However few mobiles now a day removed this 3.5 mm jack to accommodate other components of the mobile inside.