It always depends if the text content of your voice over is yours, if that is, you have the copyright on the text.
Otherwise it is a work for hire, where your right to the phonogram is purchased for both private and commercial use, does not change much.
If you charge for the commercial usage license, at least you must have written the lines, in addition to this you can sell the copy right and this type of sale is more profitable and is a surplus to a simple commercial license because it gives the buyer the full economic use of the audio and the lines or melody composed (such as publishing rights).