And sink from high to low, along a scale Of awful notes, (lines 2-3)
The speaker imposes his own feelings by calling the scale of notes "awful," a very strong adjective to use. It is the first time we understand how he really feels about mutability.
A musical but melancholy chime, (line 4)
"Melancholy" is like sadness, but, well, more Romantic. Melancholy is not just a passing imagine that you feel, say, when your pet dies. It is a spiritual attitude toward the world. A good, attentive person will naturally feel melancholy when faced with the idea of dissolution.
Which they can hear who meddle not with crime, Nor avarice, nor over-anxious care.(lines 5-6)
To understand this line, imagine this really sad, heartbreaking song that you’ve been listening to over and over. You play the song for a friend, but he’s like, "That’s so depressing. Play something light. Play something upbeat." And you’re totally amazed that your friend just doesn’t get it. Your friend would be kind of like the people described in these lines.