Explanation:

- Option:1, that it is difficult to understand: This segment is grammatically correct as it clearly expresses the consequence of the low voice.

- Option:2, what she is saying: This part is also correct. It properly conveys the content that is hard to grasp.

- Option:3, My mother speaks: This introduces the subject and verb of the sentence correctly.

- Option:4, in so a low voice: Here's the error. The correct phrasing should be "in such a low voice." The word "so" is not used directly before "a" in this context.