You can say, "I'm auditing this course." This has a very specific meaning, at least in American English. This means you are a student in the class from beginning to end but you are not receiving a grade. Sometimes local people or older people will audit a university course for their own personal improvement. You could also say "I'm just trying out this class" or "I'm sitting in on this class." This means you are trying the class for one day to see if it is right for you. If it is, then you will take the class and receive a grade. You can also say "I'm trying out this class to see if it's right for me." If it's a language class you might say, "I'm trying out this class to see if it's the right level for me." I don't think "audition class" is right, or at least that isn't something we say in the U.S.

The answer provided by 牛肉面 is satisfying, isn't it?

I'm here for the audition classes.

I'm here to make a try at this course.

I'm here to have audition courses.