In general, examples of mobile devices that can output visual and sound signals include smart phones, tablet PCs, and MP4 players (including MP3 players). 
A smart phone refers to a complex wireless communication device in which advantages of a mobile phone and a personal digital assistant (PDA) are combined.  A PDA function is generally added to the function of a mobile phone in the smart phone, by which interconnection of PCs, management of personal information, wireless Internet, transmission and reception of faxes as well as voice communications may be supported.  The smart phone supports both keypad and electronic pen input methods so that letters can be input more conveniently, a large capacity memory can be employed as compared with a mobile phone, various programs and data can be used due to an operating system thereof, and programs can be constantly added or deleted. 
Meanwhile, a table PC is a new concept product in which a portability of a PDA and the function of a laptop computer are combined and has a touch screen therein, so that programs can be executed by using a pen.  The tablet PC having a function of directly recognizing the letters written by a user and storing the letters as data allows a user to use the wireless Internet while having the function of a desktop computer.  Further, since the tablet PC uses the Microsoft Office program which an application used in general PCs, a beginner can use the tablet PC without difficulty. 
Meanwhile, the tablet PC is a player device that supports a file format defined as part 14 of MPEG-4 which is the standard of video compression encryption of ISO/IEC JTC 1. 
The mobile device (hereinafter, a mobile device includes a smart phone, a tablet PC, and an MP4 player and in particular, refers to a smart phone) includes a screen (OLED unit) for outputting images such as videos, images, and letters (numbers) and a receiver (speaker) for outputting sounds such as such as voices, and a microphone for inputting sounds is provided in a smart phone and a tablet PC. 
In a description of an external structure of the mobile device, in general, as shown in FIG.  1, a predetermined area for a sound processing unit, that is, areas for a sound output unit 3 and a sound input unit 4 are secured at upper and lower ends of a front surface of the body 1, respectively, and an image output unit 2 is disposed at an intermediate portion thereof. 
Since the conventional mobile device should secure areas corresponding to 10% to 15% of upper and lower ends of the front surface of the body 1 for the sound output unit 3 (also including a camera module in addition) and the sound input unit 4 (also including a simple input unit in addition), respectively, only a maximum of 70% to 80% of the front surface of the body 1 can be secured for the image output unit 2. 
Further, since demand calls for a large sized image output unit 2, the area for the image output unit 2 must be enlarged thereby also enlarging the body 1 and therefore increasing the size of the mobile device. 
Furthermore, the sound input/output unit of the conventional mobile device is coupled by a double-sided bonding unit having a cushion function.  However, in the conventional coupling structure, a bonding force may be lowered according to a change in an internal temperature condition of the mobile device or the sound input/output unit may be separated from a fixed object as the bonding force becomes weak. 
In addition, in the coupling structure of the sound processing unit of the conventional mobile device, a screw passes through a screw fixing member protruding to the outside of the case to be coupled to a portion that is to be fixed.  However, the conventional coupling structure directly increases a volume or an area of the sound processing unit, increasing the size of the mobile device which has a limit in space.