They said in the keynote that they worked to make the speakers sound great. Supposedly to even have some bass....which I will be interested to hear. They tended to be rather tinny...so it will be cool to see if it really is audible.
I spent some time last night comparing the speakers in my 2011 17" to my new rMPB. 17" sounds way better... but the 15" rMBP is good compared to most other 15" laptops.
Not surprisingly, the 15" outputs almost nothing at the lower frequencies - but I think it may sound a little better @ higher freqs. Unfortunately when you compare the two side by side it's a pretty clear win for the 17".
After listening to the 17", it sounds like the rMBP is missing about 20% of the freq range.
(FWIW I did a blind test with 4 different people that were around at the time.)