If you find the music is low too, it may be a contact issue. The design of some 3.5mm jack plugs coupled with the recessed 3.5mm Socket on the phone can mean that the jack plug doesn't make perfect contact with the counterparts within the socket, resulting in low volume. Put simply, they don't always fit in all the way.
Has it got 2 plastic bands around the plug or 3? If its 3 its a TRRS connection, which I have never heard of lo volume before. Usually they are TRS (2 bands) but then not all.