Reread the first stanza (lines 1-22). to what does the line "with a soft inland murmur" refer?
the wind over the hills
the water in the mountain spring
the gust of wind that passes over the speaker while he is looking down on tintern abbey

the water in the mountain spring

Explanation:

At the very beginning, the poet talks about the water from the springs that he hears for the first time after five years. As he is revisiting the place after these five years and he describes the waters that roll from these mountain springs as having "soft inland murmur".

In the next lines, he describes then the other things he sees around in the nature that he has not seen in five years - skies, cliffs, ground, etc.