When your PCs enter the woods, are the woods quiet? Maybe too quiet?
Are your jungles quiet and sound-suppressing except for a few monkey noises?
Are your prairies and hills lacking noise until the wind kicks up to signal danger?
Yeah, mine are too.
One thing I get reminded of whenever I'm back in Japan (like right now) - nature is really, really noisy. It's not car noises and fire trucks and neighbors fighting at 2 am. It's not the whine of engines and the buzz of lawnmowers at the exact minute they're allowed to start working.
But nature? Nature is cricket noises, cicadas buzzing, bugs and beetles, birds chirping, animals in the undergrowth moving around. Cows lowing, horses nickering, trees creaking in the wind.
Unnatural noises can be louder - much louder - but the idea that nature isn't a din of continuous noise and action is easy to think if you live in the city or suburbs. Natural surroundings seem like they'd just lack the man-made noises and thus be quieter.
No such luck.
For example, the cicadas in Japan are astoundingly loud - so much so you have to flee indoors to get a respite from the noise of the trees in the suburbs. (Sadly, I don't have the video I took a few years back on a small island off Shikoku, Japan - it's just a nonstop drone of insect noises meshed together. It's on another computer or I'd post it here.)
I think some of this also comes from fantasy fiction - the woods are always quiet. The caves are always dark and quiet. The outdoors is a place of peace, serenity, and bucolic calm even in the presence of humans. One fleeing adventurer steps on a stick or a leaf and alerts the entire woods to their presence and gives them away to the honed ears of the pursuers. In other words, nature is always -0 to hear anything.
In my own games, even when I mention noises I don't always keep mentioning them. It happens once, then I let it go in my distraction doing everything else.
Maybe I need to make a general "noise and disturbances" number and put it up on the GM screen. It's -2 to hear, here, for everyone. You make a normal Stealth check, but your opponents get a -2 to their Per roll to hear. Putting it up on the GM screen or out on the table would mean it's easy to remember. It'll also make it clear how painfully easy it is to be heard underground when that becomes a bonus.
By the way, I'm all for Survival being a constant Complementary Skill check for Stealth while outdoors. Make both rolls, always - the wilderness newbie doesn't sneak as well as the experienced survivor.
In short, though, nature is noisy. Don't let all of your woods be quiet and your grasslands peaceful. Add some noise - it's realistic and makes the environment matter more.