5. I am listening to someone singing.
我在听人唱歌。
还能这样说：
Listen, there is someone singing.
I hear someone singing.
谚语：
When the owl sings, the nightingale will hold her peace.
枭鸟唱歌，夜莺屏息。
6. The singer was singing to the guitar.
那个歌手正和着吉他唱歌。
还能这样说：
The singer sang while playing the guitar.
The singer was singing with guitar accompaniment.
谚语：
The swan sings when death comes.
死期来到，天鹅歌声美妙。
7. If there is time I will sing all the songs that I learned.
如果还有时间的话我会把我所有会的歌都唱一遍。
还能这样说：
If time permits, I wish I could sing all the songs that I learned.
With enough time, I will sing all my learned songs.
应用解析：
ahead of the times 跑在时代前头；
ahead of time 提前；
at a time 一次；同时；
at times 有时，偶尔；
behind time 误时
8. My singing is out of tune.
我唱歌跑调儿了。
还能这样说：
I sing out of tune.
I sing off key.
谚语：
There's many a good tune played on an old fiddle.
老当益壮。