Hi there,
I'm not an expert on the plants that will grow in your area, but do have all sorts of opinions about rocks...
I think if this were my project, I would do what
@GBBUDD said regarding creating pooling areas to break up the flow, which would not only vary the sound, but also slow the water down and give you more control over the splashing. In addition, I would take all of the rocks that are currently in a vertical position and lay them flat as seen from your viewing position. The idea is that the bedding plane of all the rocks would be more or less the same. I'd also use those amazing large rocks as spillway rocks. You've got one on the right that tapers a bit to one end. That tapered bit would be great as a spillway, with the fatter end serving as a frame rock.
Since it's hard to visualize all this, I've done a quick bit of photoshopping, just to move a few rocks around and show the result. Here's what it would look like with the two larger rocks laid flat and used to create a pooling area and spillways. I've also moved around a few other rocks upstream to make the stream zig-zag a tad more. The more rocks you lay flat like this, the better!
Here's the before and after with just a few rocks laid flat...
View attachment 147942
You could do a couple of these pooling areas / zig-zags along the stream with the same width of liner as you have currently. You can also bury some of the remaining large rocks and use them to build retaining walls off to the left and right, thus naturalizing the whole feature and fitting it into the landscape.
Then just add all the great ideas for plants that folks have recommended, and vioila! Instant paradise!