For future people in a situation like this, you should use flowable fill, it is much better and gets you very close to 100% compaction without any manual compaction. The local concrete company would deliver it and pour it in. It fills voids nicely and when it is hard, you can still dig it with a shovel. It is essentially sand with a bit of cement.
In your current situation, you could dig down a ways, add water as they said and then compact it as best you can in 4" lifts. Most likely 4" will be fine with voids under it, but that hollow sound would drive me crazy. As Tink said, you could always pump grout in through holes when (or if) it gets a void under it. I would drill them afterwards instead of leaving access holes. Just make a designated area that you know there won't be rebar to hit with the drill.