I am trying to amplify the music signal coming from a phone using an LM386 module for a 8ohm 3W speaker using a 5V supply,but when I connect it to the source,it amplifies a portion of the music,not all of it.The instrumental/background music can be heard,but not the lyrics(they are not audible). Please help.
You've got the input wired up wrong. What you should be doing is using two amplifiers; one for left and one for right but instead, you are taking the left and right signal (ignoring the common ground conenction) and feeding that to your amplifier. Because vocal and most bass stuff appears equally in left and right channels on most popular music, in effect, there is no net signal produced when you connect left to the input and right to the amplifier ground connection.
You get a similar effect on stereo speakers when one is wired in reverse - at certain points along the axis between the two speakers, bass sounds can dissapear. Many audio tools provide this feature too.
If you want to fix this try using two 1 kohm resistors to join left and right channels into one input and use the regular ground connection from your music source to connect to ground of your amplifier.
See this youtube video that explains what is happening when you invert one channel and add it to the other (which is basically what you are doing inadvertently).