Use the adjective melodious to describe something that sounds like music, like a babbling brook or a little boy's soft humming.
Anything that makes a pleasant, tuneful sound can be called melodious, which makes sense when you spot the word melody in melodious. Both have a root in common, the Greek word meloidia, which means "singing, or a tune for lyric poetry." It, in turn, comes from melos, or "song."