What is the occupation of that kind of audio novel? Where can I find this part-time job?

Well, it should be called seiyuu, which is cv.

If you want to be a cv, you must have a good voice. If you practice pronunciation, you can find some professional seiyuu videos online.

You can publish some works and sign your name on some original video websites (such as Bi Li) and some music software (users can upload their own voices, such as Himalayas). After accumulating some popularity, you can contact some companies to sign contracts.

It can also be posted on social networking sites, such as Weibo.

Most of this industry is professionals, but there are also many part-time jobs. Come on!