How can i make the volume louder on calls i barely can hear the other person on the other end.

On the left hand side, there is a slider and two buttons below it, press the upper button to increase the volume. If that doesn't help take it to an authorised apple centre and let them help you.

maybe by mistake you reduce the volume on the iphone.

try the following during a call press on the right control up volume to see if that improve your sound.

if that doesn't work you should go to an apple store or your cell provider.

this phone has many different volumes to play with, one of them is the volume for, when your calling someone, simply, when you call someone, press the volume up button, and you should hear the person louder

On most mobile phones this is a setting hidden somewhere in the "Settings" or "Options" menu, but the easiest way to to fix it is with a simple phone call. While you are currently on a call with another person, you can turn the speaker volume up using the same button to turn up the ringer volume.

Note that this will only work if you try to turn up the volume while the phone on the other end is ringing or you are currently connected on a call.

while on the call use the right and left scroll button

when you call some one
try pressing up the button on the left
there are three buttons on the left and 1 on the right
press the up button it will say on the screen ring style volume 7 or 6 or what ever you have set it up to
but during the call when you press the same button it lowers or higher the volume of the speaker
thats the easy way to do it

You can use a FM modulator and hear it fine through your radio, but then you have the problem of no radio to listen to.........Im lost too.

The speaker needs to be replaced.
Go to a cellphone repair agent in your area.

Amplify the song with Nero Wave editor, from menu, VOLUME -> Volume change and go up to +10 db but not more.