I believe you can use them legally all you'd like if you have the appropriate edition (For example, Free Edition doesn't have some sounds that Personal Edition has, so they shouldn't be using the Personal Edition sounds).
"You are free to use all bundled music that comes with your edition exclusively for creations made in Construct 2 within the terms of the edition you are using. Please contact us if you have any questions or queries."
If you want to credit someone then it would be Scirra Ltd.
At least that is how I interpreted everything. Someone please correct me if I am wrong.