How does the narrator say that nature speaks to those who visit in their happier hours?

The answer to this question can be found in lines 3-5 of this poem. The first two lines of the poem introduce readers to a man that really enjoys nature. He enjoys it so much that he is able to hold "communion" with nature.  

To him who in the love of Nature holds   
Communion with her visible forms,

Notice that "Nature" is capitalized. This alerts readers to the idea that nature might be more than a thing. The narrator might be telling readers that nature is a person. That suspicion is confirmed at the end of line 2 when the narrator says that "she" (Nature) speaks to him during his times of communion with her.  


To him who in the love of Nature holds   
Communion with her visible forms, she speaks   
A various language



The narrator then goes on to explain how Nature speaks to him. He explains that Nature responds differently to him depending on his mood. The first mood he explains is what the question is asking about. In his happier, "gayer" moods, nature speaks to him with a smile and gladness. She is also beautiful and eloquent. 


for his gayer hours   
She has a voice of gladness, and a smile   
And eloquence of beauty,



Basically, when he is happy, nature is happy. He then goes on to say that when he is sad, Nature responds with sympathy and helps him feel better.  


and she glides   
Into his darker musings, with a mild   
And healing sympathy, that steals away   
Their sharpness, ere he is aware.