Suddenly I sang off key.

突然间我跑调了。

Her singing was off key.

她唱得走调了。

You will butcher that song by singing off key.

你会因为唱走调而糟蹋了那首歌。

His friends tell him that he sings off key but he doesn't care.

他的朋友告诉他说他唱歌走音，但他并不在乎。

I am going to sing for you, a little off key perhaps, but I will sing.

我将会为你歌唱，也许有点儿跑调，但我依旧要歌唱。