There is no volume control app, at least not without jailbreaking.
There is, however, a trick that might help you with finer control of the sliders you already use. Instead of sliding with your finger on top of the slider, where it obscures what you can see; touch the slider and slide/move your finger down until you can see the slider better, then move the finger right/left to adjust.
Also, music/audio apps almost always adjust the iPad's sound, not just for their own apps. So if you can find an app with controls you like, you can use that.
For instance, if you play a song in the Music app and choose the Playing Now option, you get an album cover view with a larger than usual volume control under it. Adjust the music to the level you want, then stop the music. When you go back to the home page and check the Control Panel you will see the volume changed there as well.
That's about all I can think of.