What tone is set in the opening exchanges between the characters?

There is a sense of dramatic irony that is kind of humorous even though the character doesn't intend to be. For example, Arthur Birling, who appears wise at first, sounds rather silly as he lectures his son. All his predictions about famous events turn out to be wrong, "the Titanic-she sails next week...and every luxury- (liner) is unsinkable, absolutely unsinkable." The humour of this via dramatic irony gives the reader a sense of dark comedy about these oh so proper people.

The tone is very much one of relaxation and comfort. Sheila makes a friendly joke at Birling's expense, comparing him to a 'purple-faced old man', which shows that there is no tension in the atmosphere and they do not have any other concern bothering them. Gerald admits that he is a bad judge of port, showing that he is comfortable enough to engage in light hearted small talk without having to watch what he's saying. Sheila's first utterance is described as delivered 'gaily', showing that she is happy and content, a little excited. Also a few lines later she refers to Mrs Birling as 'mummy', which I don't think she would do if she was not totally relaxed and happy in the situation, showing that the tone at the table in the first few exchanges in very light-hearted and friendly.