Good luck with that - they are very similar! There ae a few differences, though: Viola odorata is not as commonly found as Viola sonoria, and is smaller, with leaves 1-3" wide compared to sonoria's 5" leaves. Its leaves are more rounded, smoother, and darker green, and its flowers are purple/lavender only. Sonoria can have Purple/lavender, pink, blue, or white flowers. Sonoria has a clumping habit, while odorata clumps, creeps, and spreads. And, of course, odorata has a sweet perfume.  

Hope this helps!