Accent difficulty should not be the main progression.
A higher level should not mean unclear accents, exaggerated accents, or voices that are hard to understand. That could make the audio feel unfair or stressful.
A better system is gradual voice development.
For lower-level learners, the voices should be clear, gentle, and easy to follow. The learner needs safety first.
For middle-level learners, there can be more voice variety. This may include male and female voices, different international accents, and more natural rhythm.
For higher-level learners, the voices can become more natural, with less support. The topics can become wider, the rhythm can feel closer to real exam situations, and the learner can become more comfortable with unfamiliar voices.
So the progression is not about making accents harder and harder. The progression is: clear support -> wider variety -> more natural English -> confident recovery.
This is a healthier and more realistic way to prepare.