What is 'sound evidence'
The enormous sales figures all over the world are sound evidence

Question

Answer

The adjective 'sound' here means 'safe' or 'reliable'.

As an adjective it can also mean 'in good condition'.

The foundations were sound.

It's also British English slang to mean 'very good' or excellent.

'The new song by Oasis is sound (as a pound).'

It can also mean severe.

'Manchester United took a sound beating at the weekend.'  [They lost heavily]

Answer

The meaning of "sound" here is "healthy/stable/solid".