Did you try the 3 R's?
Basics from the manual are restart, reset, restore
Follow the 3R's- if a restart does not solve your problem proceed
User troubleshooting is:
1.restart, ie power off/on,
2. reset http://support.apple.com/kb/HT1430,
3. restore from backup, restore as new device. http://support.apple.com/kb/HT1414
I have found a temporary fix for this problem.
1) Connect Iphone to Itunes
2) Press CTRL + A to select all audio files
3) Press CTRL + I to open file information window
4) Go to options tab
5) Increase the volume to +100%
6) Select equalizer as CUSTOM
Once this is done all the audio files on your phonre will be updated and the volume is all good