Musisz looks like a loanword from the Germanic languages, therefore it's easy to memorize (ich muss/I must)
Sounds fine to me, but learners often have problems distinguishing between ś/si and sz.
Yes really the all sounds in the whole course are so fast for a new begginer, sometime it is hard to differentiate between the endings, here it is an example.Can sounds be slowed?..
You have a slow audio option in the hearing exercises, and I guess you can also hover over specific words and it will be slower than usual.
I think the English translation of You must be strong should have been accepted