This is the 51st most frequent Chinese word.
所
A particle or noun modifier indicating "place," "institute," or "that which is."
In this sentence, '所' is again part of the '所...的' structure, showing the action '听过' (has listened to) which qualifies '音乐' (music).
他所听过的音乐让我感动。
The music he has listened to moved me.
In this example, '所' is used as a measure word for institutions such as schools or buildings, preceding '著名的学校' (famous school).
那是一所著名的学校。
That is a famous school.
Here, '所' is used as part of the structure '所...的', which acts to nominalize the verb that follows it '知道' (know). It introduces a relative clause describing '全部事实' (all the truth).