folk song的音标？

For an encore, he sang an unaccompanied folk song.应观众再唱一曲的要求, 他清唱了一首民歌.She is humming a folk song that I never heard before.她在哼一首我以前从未听过的民歌.She was asked to sing a folk song by a large audience.许多观众要求她唱一首民歌....

...

folk song的复数形式为：folk songs...