Greetings Tezuka_Watanabe,
I see you're looking for help with adjusting the audio of a show that you purchased, so let's see how I can help out.
For starters, we'll want to be sure that the show you purchased has the desired audio available to be changed. When viewing the information of the show, you may see that there are two different locations showing the language so we'll need to pay close attention to that. Some shows and movies will display "Audio" under "Languages" as well as the "Original Audio" that was available when the show was produced under the "Information" section. With that being stated, the languages available would only be what's listed under the "Languages" section. For assistance with checking this information out, take a look at the section labeled "Learn about a movie or TV show" of the following resource:
Buy movies and TV shows from the Apple TV app
If you see that the show or movie lists an additional language that you desire as available under the languages section, check out the next resource for assistance with changing the language.
Change subtitles or languages for videos in the Apple TV app or iTunes Store
If you're unable to change the language while you should be able to, Contact Apple Support to look into this in more detail.
Cheers!