That's beyond the scope of this copyright sub, but I'll take a swing. What they are going to want in the business proposal is a proposed royalty on sales, and an advance on that royalty; and they will propose (or expect a proposal for - I am not in this business and don't know the standards) a bunch of secondary stuff like approval rights, audit rights, regular sales reports with as much detail as they can get you to supply, and the right to terminate the agreement with as little disadvantage to them as they can negotiate. You will propose the term, and the territories in which you have the audio book rights. You will propose how you plan to create and then market and sell the product. They might want your low/medium/high sales projections.
The advance is to make sure they aren't wasting their time with you, and to make sure they make money even if your sales are zero.
I own but have not read one of the editions of This Business of Music, as seen in School of Rock. It is not exactly in this business but is in the ballpark; maybe it will help.
You will need an attorney to review and help negotiate, if you want, the contract; this will be complicated by the fact they will want the contract to be in Japanese, governed by Japanese law; while you may want it to be in English, governed by the law where you are.