When working with lesson audio it would be useful to be able to make the player repeat every sentence several times before going to the next.
It would help with listening: the first time around one remembers the overal gist / structure of the sentence, on next repeats it is possible to focus on individual phrases, words and difficult places.
It could also be used for “shadowing” exercises, where the student repeats the sentece after the example. A beginner would start after the sentence has ended and would often speak slower than the example, so would need more time between repeats than the actual sentence audio length. An intermediate / advanced learner could learn to start speaking while the example is still playing and speak at the same speed as the example or even faster, so would want less time between repeats.
The desired number of sentence repeats would also vary depending on the student level / personal preference / particular text.
For the player UI I would consider adding a dropdown menu to the current repeat button (with choices e.g. of “no repeat”, “repeat all” or repeat sentence: “2x”, “3x”, … up to “6x”), plus an extra dropdown for the silence length in % of sentence length like 50%, 75%, 100%, … up to 200%. The silence length has of course no effect when repeat is set to “no” or “all”.