唱歌, 走路 and 谈话 are not verbs. They are phrases (verb + noun). They can act as nouns. For example, 唱歌 can mean "sing a song" or "singing". -你能为我唱首歌吗？ -Can you sing a song for me? -唱歌是一个放松心情的好办法。 -Singing is a good way to relax. I think phrases are more often used. Because phrases are self-contained. They give you a complete meaning. So they sound more natural than separate characters. For example: -他在唱。 -他在唱歌。 -他在走。 -他在走路。 I can't say the first ones are wrong. But nobody talks like that. When do we use single charaters? Here are some examples. I think most time there will be a context when we use single characters. -你听，有人在唱歌。 Listen, someone is singing (a song). -他在唱什么？ What (song) is he singing? two people are walking, and one says to another: -你走快点！ Walk faster! two people are talking, and another two peole are listening in: -他们在谈（说）什么呢？ What are they talking about? You see, when we use single characters, we are often refering to some specific thing. While the phrases have general meanings. They don't have to be in a context. They are self-contained.

In your example,there`s no big different,just oral Chinese,or a kind of shortcut,I think.