Leomi M. wrote:
> I have a question, if someone could help me?
> Why is bandwdith of voice from to ?

Evolution made human voice operating in a typical frequency range.
Depending on being a man, woman or young kid the "center frequency" is 
around 125 to 440 Hz.
Typical "bandwidth" of human voice is roundabout 1-3 octaves around this 
"center frequency".
Taking into account that our voice is also containing harmonics which 
are essential for speech perceivability, 12 KHz is commonly understood 
as upper frequency limit for speech.
In order to save bandwitdh (e.g. to closely space radio transmission 
channels) a lower limit could be used (e.g. 2 KHz), which is sufficient 
for speech, but sounds a bit "muffled".

> Why is bandwdith of music 50Hz from to 15kHz ?
That's what our ears typically are able to hear.
Young human beings can hear frequencies up to 20 KHz, so "20 Hz - 20 
KHz" is also a common definition.
Music instruments are actually producing frequencies above 20 KHz as 
well, but as we don't hear them anyway, we don't have to take care for 
these frequencies when building devices recording or broadcasting music.

> Why is bandwdith of video signal from 10Hz to 5MHz ?

10 Hz is probably wrong. Required bandwidth is "pixels x frame rate".
5 MHz is at the lower end. Standard resolution would be 720x480 pixels 
by 25 Hz (PAL), which equals to something around 8.6 MHz.

> And also at what frequency is usually human voice, music(voice) sampled and why?

Compact disc uses 44.1 KHz sampling frequency, which results in an upper 
audio frequency limit of 22.05 KHz (half sampling rate).
This is commonly accepted to be sufficient for musical purposes.