"was heard the mingled measure" (line 33)
"Measure" is a very rhythmic, musical idea. The phrase "mingled measure" turns the different sounds of the river into a complicated piece of music. It gives us the sense that all that roaring and crashing comes together to make a kind of harmony. In this poem, music comes from nature just as much as it does from mankind.
"A damsel with a dulcimer" (line 37)
This is the first place where music is specifically mentioned in the poem. Having that dulcimer lets us know that the speaker's dream of this damsel is filled with sound. He's not just seeing her, but also hearing her play and sing. Like we saw with the mingled measure, music has been under the surface in this poem, and now it rises to the top.
"Her symphony and song" (line 43)
Here the speaker tells us that he wants to bring back the vision of the damsel. But what he misses is not her words. In fact he barely tells us what she said, except that she was singing about some mountain that we don't recognize. What he misses is the feeling of the music. When you hear a symphony, you don't get specific information in the form of something you could write down or describe. You get a mood, a series of feelings that touch more than your brain. This is what Coleridge is trying to do with this poem, to give you the emotion of the vision as much as a description of what is in it.