Level of competence

Should we totally master each audio session prior to moving on to another?

It depends on what you understand by "mastering an audio lesson". Yes, it is indicated that you know well everything that you learned new in the lesson, but there are some things that could be left off and practiced later. For example, at the premium plus course, at a certain point I just couldn't master the Japanese sounds and their meanings, but while I proceeded with the course, I eventually got to practice them and reuse them often, so it helped a lot.