1. I'm installing 1/2" engineered bamboo click lock type flooring. I don't want to do 1/4 round on the baseboard I like the look of the baseboard meeting the wood directly so I'd like to leave the expansion gap under the baseboard. Tthe baseboard is only 1/2" thick so could I leave 1/4" at each end and be safe to allow for expansion and contraction. My longest run is 30' but that's only a 3' section. I'm not sure what difference this makes but this is the main floor of my house it's over 3/4", underneath is a finished basement and we use A/C all summer. Depending on your recommendations if I have to I will cut the bottom of the drywall up the wall so the floor can expand under that as well.

2. Which direction does the sound barrier underlayment run? parallel to the joists or the flooring?

I have included a link to a crude diagram of my flooring plan
http://www.wheelerhome.com/gallery/main ... emId=13507

Factories or mills always say 3/4" its not usually nessesary. with your idea of leaving a 1/4 inch is fine, but do it while you have some moisture in the air, come with things dry and shrink. at about 66% to 67% humdity and maintaining that year round the floor would never move. Doing it now, let it aclimate 2 weeks. then install it with your oridanal idea of a 1/4" all the way around that totals 1/2" only a 1/4" from specs once you cover your floating floor or the edges rather come winter it will probly shrink about a 1/4" making fr the 3/4 factory calls for. come summer time at it worste even without airconditioning on it will expand almost tight back to the wall. no matter how you look at it the trim shoud keep it cover to atleast an 1/8" no problem.

I did my 1000sf bottom floor in the click-lock type flooring and it came out great... for the most part. It was really hard to walk the thin line of leaving just enough gap that the baseboards covered it yet the gap was big enough for expansion. Problem is not only buckling if you leave too little, but unsightly gaps if you leave too much, and a common baseboard is a pretty thin target once it's nailed down. If I could do it over, I would have cut the drywall out and made the fitting process that much easier.