sing [siŋ] 读音见此链接：
http://dict.youdao.com/search?q=sing&ue=utf8&keyfrom=dict.index#q%3Dsing%26ue%3Dutf8%26keyfrom%3Ddict.index
singing ['siŋiŋ] 读音见此链接：http://dict.youdao.com/search?q=sing&ue=utf8&keyfrom=dict.index#le%3Deng%26q%3Dsinging%26tab%3D%26keyfrom%3Ddict.top
mouth [mauθ] 读音见此链接：http://dict.youdao.com/search?q=mouth&ue=utf8&keyfrom=dict.index#q%3Dmouth%26ue%3Dutf8%26keyfrom%3Ddict.index
mouths 和mouth 读音一样