Despite my avatar, I do own a 2004 XC70. I did not fully understand the question initially, so read the following addendum first.

There is no automatic setting because Volvo allows the driver to make the decision on what's best for the occupants. Actually I find the North American and other makers automatic locking systems a bit ridiculous and potentially unsafe. In most locations, even in the USA, this automatic door locking system may only be necessary in 5% of the situations. It seems that most manufacturers design for the exception, rather than the rule. Of course I'm a Canadian and we are a bit less effected by what other owners may consider to be a mandatory feature

If you simply want to know how to lock or unlock the doors while driving, simply press the lock/unlock button on the driver's side switch array in the door (behind the window switches). You'll also see the child proof setting to prevent the back doors from being opened.
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Cam a.k.a. CVOLVO.COM