It does sound like the front bearings. I've had to replace the left one twice on my S70 (160k mi). The sound is just as you describe. You can test by turning. If the right bearing is bad, the noise will increase in a left-hand turn and vice versa. The front bearing hub is around $150 - can't just replace the bearings.

Here's a link for the replacement procedure:

http://www.volvospeed.com/Repair/FWDbearingHub.php

How many miles on it? Sounds like it might be the front bearings. They often start making noise at 45mph or so, but mine started making a low pitched growling when moving and it turned into a whirring noise at higher speeds. Usually only one goes at a time (though the other isn't far behind). When driving and hearing the noise, try turning the wheel slightly to the left or right to "unload" each front wheel to determine which it is. If you turn it to the right and the noise lessens, then it is the right side, and vice versa for the other.

Seems that these go around 150K unless you have had an accident or hit something with the suspension.

May be a wheel bearing. I noticed such noise during a trip in Canada - thought it was some kind of winterized road surface. But it gradually (over a couple thousand miles) got worse to the point where it was obvious on various types of roads.

If you didn't do it, see if it is tied to road speed or engine speed (downshift or coast). Also does the sound change when you turn left or right, or speed up vs coast? If that happens it may be a CV joint.

That said, do the tires show any unusual wear pattern (feathering along the edges, etc)?