I read this method in Gopinath Kaviraj ji's book. It was like that when you chant a mantra, you should also listen to it, your mantra chanting may turn slow but it is worth it! Your concentration will improve. Actually it creates a cycle, the mantra you chant comes back to you when you listen to it. And while doing this your mind doesn't wander elsewhere. With this technique you may chant vachik, upanshu or mansik japa, whatever you prefer.