Have you gone to Settings>Music>Volume Limit to see if the maximum volume has been limited. Removing the limit will make it sound louder in headphones but could damage your hearing if you turn the volume all the way up.
If you're looking for an app that can raise the volume if the internal speaker beyond the maximum normally allowed, I don't believe there's such an app.
You mean beyond the maximum in settings, as in my earlier post? Is that older app in your App Store purchase history? If so, you should be able to re-download it.