To be sound: Irish slang. To be sound is a state of existence. It usually refers to someone who willingly does a favour, asked or not, although usually unexpected - Like when you ask for one rollie and your given two, or the bouncer to an over 21s let's you in when your 20.
Alternatively, "sound" can be used as an adjective to describe a genuine, nice person.
-Man, can I have two of those Rowntrees?
-Here, you can have the rest of the packet
- AW, sound