In Harry Potter and The Chamber of Secrets, what tone of voice does Tom Marvolo Riddle say...
"You'll be with your dear Mudblood mother soon, Harry"
I've read the book over and over again and it isn't mentioned.
I assume you're referring to this bit of the book
‘So ends the famous Harry Potter,’ said Riddle’s distant voice. ‘Alone in the Chamber of Secrets, forsaken by his friends, defeated at last by the Dark Lord he so unwisely challenged. You’ll be back with your dear Mudblood mother soon, Harry … She bought you twelve years of borrowed time … but Lord Voldemort got you in the end, as you knew he must.’
Under the circumstance, I think we can assume that he was using a mocking tone and practically spat the word "mudblood" given his known animosity for anyone who's not blood pure.
In Half Blood Prince, Voldemort speaks again about his feelings about mudbloods, this time with "anger and contempt":
...last week Professor Burbage wrote an impassioned defence of Mudbloods in the Daily Prophet. Wizards, she says, must accept these thieves of their knowledge and magic. The dwindling of the pure-bloods is, says Professor Burbage, a most desirable circumstance … she would have us all mate with Muggles … or, no doubt, werewolves …’ Nobody laughed this time: there was no mistaking the anger and contempt in Voldemort’s voice.
And in Deathly Hallows, he gives pretty much the same speech as he did in Chamber of Secrets, this time in a faux-cheerful self-congratulatory tone described as "jeering"
‘Is it love again?’ said Voldemort, his snake’s face jeering, ‘Dumbledore’s favourite solution, love, which he claimed conquered death, though love did not stop him falling from the Tower and breaking like an old waxwork? Love, which did not prevent me stamping out your Mudblood mother like a cockroach, Potter – and nobody seems to love you enough to run forwards this time, and take my curse. So what will stop you dying now when I strike?’
I think it's fair to say that although Voldemort's tone is generally described as being quite even, whenever the subject of blood purity comes up, it's something that engenders great passion in him.
Tom M. Riddle is always shown as soft spoken guy with a well mannered speech. Even when angry, his voice remains calm with slightly louder tone.
If going by movie, Tom M. Riddle and the overall scenario, in HP: The Chambers of Secrets, the voice is mixture of sarcasm and surprise. Emotionally, Tom M. Riddle is angry to see Harry Potter survive so long. Alternatively, it can also be suggested that kind of voice where a person has been preparing for a speech for long time and finally delivering it as he/she had expected.