address issue 1929 Ubiquitous Sound Effects, fixes a bug in determining which units are onscreen, also plays distant items softly if at all 

git-svn-id: http://svn.wildfiregames.com/public/ps/trunk@13439 3db68df2-c116-0410-a063-a993310a9797
Merge remote-tracking branch 'remotes/trunk'