It's a good idea and something I will be interested in using

However, I was very confused by the interface at first. I started a with a 3 gap exercise. I dragged what I thought was the correct word into the gap. Listened again, changed my mind but I couldn't drag in my new choice. It was a while before I realised that the correct word had been inserted for me. This was despite me not completing the other gaps.

It would be better if the answers weren't given until the user submits the answer.