If a person is doing meditation without using his hands as management tools probably he is just sitting there doing nothing, perhaps day-dreaming too, or if he is listening to some soft music, he would probably be enjoying his music too. And if he is doing mantra at the same time, probably he is doing something very similar to a singer practicing the resonance of his vocal cord - head tone, chest tone, or abdomen tone...
What I'm saying is, doing hand positions should be the first physical act one should always attend to before use music or mantra to assist one to go into the meditative zone. Actually if one can do well with chi-management using hands in co-ordination with chi-points inside the body (like the third eye and the dantian), music and mantra will not be necessary at all. Besides, for music lovers, any pleasing sound will likely to drag us back towards full consciousness.
The first principle for the hands is that the fingers (and palm) must be slightly stretched, like in zhan zhuang. It is the principle of creating gradient (yin and yang) followed by the process of combining the opposites under a meditative mind. And the body must be relaxed, yet upright, so that the central line can be maintained to facilitate chi-flow.
The next question is: where to put one's hands. There are two broad options: at centre or on the sides. Depending on one's particular meditative objective at the moment, either one can be used. When you want to establish your central line, you should fold your hands at the centre. This is also needed when microcosmic circulation is practiced, because in this position one's chi can be energized with enough power to travel up the spinal cord. And this position is the position for deeper meditative practice (i.e. post-microcosmic).
Most people begin meditation with hands by the sides resting on both knees. This is similar to the hand position of standard zhan zhuang with the hands resting to facilitate meditation. This is an excellent position to help generating chi and balancing the chi-system. Assuming that one has built a moderately steady centre line, relaxing the hands on the sides can help strengthen the body (fundamentally muscular in nature) system on the weaker side of the body. And using the body's internal strength this way, mediation can be a powerful tune up tool for aligning and balancing the body's bone-cum-muscles structure (e.g. if you find jogging hurts your knees, mediation can deliver similar results without the side-effects).