I was on OS Legacy yesterday and heards birds when I joined and throughout the game. Is this new? Or have I just been ignorant of the subtleties of this map?

Not new, been there since the map came out as far as I know.

yeah, usually the dang horn is blowing too loud for you to hear the birds in jungle (i hate that horn). i mostly play with sound effects off, you still get all the recycle sounds, but distracting sounds like then you get hit with PPCs and racs are gone.