Green’s Dictionary of Slang

a general excl. of approval, i.e. excellent! fine! no problems!

1969		T. Murphy A Crucial Week in the Life of a Grocer’s Assistant (1978) Scene v: Sound, Miko. You won’t forget?
1998		K. Sampson Awaydays 10: ‘They’re bang up for this one. If you think it’s going to be a doddle, you can fucken think again.’ ‘Sound. I hope there is hundreds of them.’.
2001		K. Sampson Outlaws (ms.) 71: Sound. Where will I find him?
2012		J. Fagan Panopticon (2013) 29: A butty in bed and a book, sound!