你的对话中的 Sounds great to me 可视为省略了主语 it。即原句等于：

It sounds great to me.

大家知道，在通常情况下，英语句子的主语是不能随便省略的。但在口语中属以下特殊情况时可以省略：

1. 省略第一人称代词，一般是I。如：

Beg jour pardon.

Told you so.

Wonder what they’redoing.

Hope he’s there.

Don’t know what to say.

Think I’ll go now.

2. 省略第二人称代词。如：

(1) 省略陈述疑问句中的第二人称主语。如：

Turned out all right?

Want a drink?

(2) 省略附加疑问句前面陈述句的第二人称主语。如：

Had a good time, did you?

Want a drink, do you?

3. 省略第三人称代词he, she, they。如：

(He / She) Doesn’t look too well.

(He / She /They)Can’t play at all.

4. 省略代词it。如：

Serves you right.

Looks like rain.

Doesn’t matter.

Must be hot in Panama.

Seems full.

Makes too much noise.

Sounds fine to me.

Won’t be any use.

5. 省略 there。如：

Ought to be some coffee in the pot.

Must be somebody waiting for you.

May be some children outside.

Appears to be a big crowd in the hall.

（根据夸克等人的《英语语法大全》整理）