I don't doubt that they heard it, but by the time it was blowing Frodo and Sam were already firmly convinced that they had to leave the fellowship, and were already sailing away. I think Frodo was surprised more that Faramir heard the horn then that the horn was blown. We must remember that Boromir had just tried to take the ring by force. I found this quote in chapter ten book two, Frodo is talking to himself.

"This at least is plain: the evil of the Ring is already at work even in the Company, and the Ring must leave them before it does more harm. I will go alone. Some I cannot trust, and those i can trust are too dear to me: poor old Sam, and Merry and Pippin. Strider, too: his heart yearns for Minas Tirith, and he will be needed there, now Boromir has fallen into evil. I will go alone. At once.

So we can see that the blowing of the horn would not change Frodos mind, since he already decided not to trust Boromir, and to go alone. If he is to go alone, he has to go ASAP, and he knows that if the others in the company see him they will at least send someone with him.

the Kine or Araw was the stuff that the nicest parts of the Horn were made of. so Gondor liked to go to the Black burned fields of Rhun to get killed by millions of Easterlings to get these things was kind of wierd, but the Men of Gondor were better warriors than they were.