Hi mate have a try at doing any of the following see how u get on
1) Fix #1: while playing music through headphones, go to the Voice Memos App. Launch the App and you will notice that the music fades out and will be paused. Now you do not need to actually record any memo, just let the Voice Memo App run for 3 or 4 seconds and then exit the App. Now after exiting the Voice Memo App, go to the Music app and play the music. It should now play at proper volume.
2) Fix #2: While playing music throught headphones, go to Settings App > General > International and change the Language to Any other language. For instance, if you are in English, then change to British English or vice versa. Once you do that, you will notice that the Springboard relaunches. After the Springboard relaunches, go to the Settings App again and change your language back original language. Once again, your Springboard will relaunch. Now try the Music, it should be at proper volume now.
Also mate have you tried a restore and what was the result