Hardy makes use of a clever double entendre here. “Air” refers to both the song itself—”air” being a word for a simple tune—as well as the air through which the song passes. This is an unusually potent double entendre in that the two definitions of the word directly interact with one another.
“Bine-stems” are the stems of climbing plants. The simile of the broken lyre strings continues the overall theme of destruction and also casts an air of silence over the scene, which becomes important in the third stanza.
"Winter's dregs..."See in text(The Darkling Thrush)
This is a metaphor for snow. The image of dregs—the residue left from liquid—offers connotations of emptiness, or an end to joy. It brings to mind a finished beverage.
