I agree with pretty much everything that Ellis555 says.
It’s definitely Laertes that is being addressed. The two of them have just been fighting in Ophelia’s grave and have been separated by the attendants. Hamlet’s immediately preceding dialogue is clearly addressed to Laertes:
“I loved Ophelia: forty thousand brothers
Could not, with all their quantity of love,
Make up my sum. What wilt thou do for her?”
“'Swounds, show me what thou’lt do:
Woo’t weep? woo’t fight? woo’t fast? woo’t tear thyself?
Woo’t drink up eisel? eat a crocodile?
I’ll do’t. Dost thou come here to whine?
To outface me with leaping in her grave?
Be buried quick with her, and so will I:
And, if thou prate of mountains, let them throw
Millions of acres on us, till our ground,
Singeing his pate against the burning zone,
Make Ossa like a wart! Nay, an thou’lt mouth,
I’ll rant as well as thou.”
I also agree with Ellis555 about the interpretation of the lines following “hear you, sir”.
Finally, I agree with Cher3 that Laertes and Hamlet were once friends, or at least friendly rivals.
But Hamlet was dating Laertes’ sister, so doubtless there may have been some sort of one-upsmanship there, as it were. Recall that Claudius alludes to Hamlet’s envy when Laertes’s fencing abilities are praised (from Act 4, Scene vii):
“[A Norman gentleman] made confession of you,
And gave you such a masterly report
For art and exercise in your defence
And for your rapier most especially,
That he cried out, 'twould be a sight indeed,
If one could match you: the scrimers of their nation,
He swore, had had neither motion, guard, nor eye,
If you opposed them. Sir, this report of his
Did Hamlet so envenom with his envy
That he could nothing do but wish and beg
Your sudden coming o’er, to play with him.”
And they use this rivalry to lure Hamlet to his death.