This problem can't be fixed with the help of an application or something related. The sound settings are embedded into the kernel of the Android OS. You will need a custom ROM which has this feature especially modified. Unfortunately, custom ROMs also offer disadvantages. If you still have warranty, I recommend contacting HTC. It's also possible that the sound is limited to the maximum value you can increase it or it's a hardware problem. There is no tool to increase the sound quality or volume level.