After a lot of digging around on the web, I found out several things:
1) the answer to your question does not seem to be known with any certainty
2) the middle ears of seals are lined with a vascular mucosa that can become engorged with blood and swell as the animals dive, thus obliterating the airspace in the ear at depth, making equalization moot. In essence, diving mammals basically force the air out of their ears totally as they dive. The replacement of air by blood-filled veins not only makes equalization unnecessary, it also enhances water conduction of sound into the inner ear
3) animals, like whales, that spend most of their time underwater have very hard, almost calcified eardrums which can tolerate severe pressure differences without pain or rupture...the eardrums of seals are tougher than humans, but not as dense as whales. Thin eardrums are needed for detecting air waves, but sound waves in water are better detected by harder structures, like bone. The seal must hear on land as well as in the water, thus its ear isn't particularly great for either. From what I've read, the seals ears tolerate diving well, but don't hear very well at great depths.
