1)anything/anyone who is cool,good, okey doke, particularly in the sense of being trustworthy/reliable etc etc.
when someone/something's really, really 'sound' you say they're 'sound as a pound.' ie they're so sound, they're as trustworthy as the wonderful british currency!!! that's one hell of a compliment.
2) moaning, losing your temper over something. ( as in 'sounding off')
1) some people don't like that girl, but i think she's pretty sound actually.
- that's my best mate over there. he's sound as a pound he is.
2) she's sounding off about some crap no one cares about again!!