Definition of sight-sing in US English:

sight-sing

verb

[with object]
  • Sing (music) at sight, without preparation.

    • ‘Many voice students are incorrectly taught to sight-sing by memorizing the sounds of intervals.’
    • ‘Learning how to sight-sing starts with learning the musical notes and knowing how to sight-read them.’

Pronunciation

sight-sing

/ˈsītˌsiNG/