I'm going to take a crack at hear noise, among the most annoying, least defined and generally crappy game mechanics in AD&D ... and at the same time, absurdly difficult to improve. I've never been able to do so, I don't know of any other mechanic that does more than simply inflate the base problems - first, that the character supposedly fails the roll and can't hear something that ought to be perfectly clear, and second, that what can and cannot be heard is far, far too subjective, and therefore almost wholly dependent upon the grudging or generous DM.
What's needed is a measurement ... and as it happens, in sound there is one.
Having spent some time trying to hammer down the mechanics of decibels, I've put together the following table:
Now, while I think this idea may in fact be easy to use, once one is adjusted, initially it is a bit of a mind fuck ... so take that ol' mind out and give it a good stretch.
The numbers were put together from an online calculator, so I admit that I did not do my own math; this one was a bit beyond me, I'm afraid ... but remember, we're not trying to pass a physics test here, we just want a workable system for D&D. One where we can measure sound, not just point at it.
The feet/yard/miles columns are included so the DM can choose what particular measure is useful given the particular circumstances. The examples of sound for decibels is more or less what you'll find online, though I've worked to remove all modern references, such as rock concerts, jet planes and jackhammers. Those are very common examples you'll find on decibel sites.
How this works is this: the distance indicated is that point where the sound being made has been reduced to barely audible (12 decibels). It's that distance that the sound can just barely be picked up ... because, it must be understood, the amount of sound a thing makes is dependent on how far away you are from that sound. That's obvious ... but where is the mechanic that demonstrates it?
If you half the maximum distance, you increase the level of sound by 6 decibels (not double!). Half that distance again, and that again increases the sound by 6 decibels. This is how the decibel mechanics actually work. A sound that is 6 decibels higher can be heard at twice the distance.
So the hammer on an anvil can be just picked up at 3/10ths of a mile ... but it sounds very minute, like the clear sound of a pin dropped on a floor in an otherwise silent room. At 273 yards, the hammer sounds as loud as the rustling of leaves. At 137 yards, something like the collective sound of a nearby farm. And so on. At nine yards, as loud as a stream, at six feet, as loud as a dinner party.
Sound is such a deceptive thing, however, and the real problem with your perception of a hammer right now is that you tend to think of it as loud as it is when you're standing right next to it, actually using the hammer. It's hard, really, to grasp how loud it sounds, exactly, from nine yards away. Moreover, it is almost impossible for you to get a good grasp on how loud the rural countryside is, or a 'quiet garden.' That's the mind fuck. Every idea you have about the relationship between sound and actual recognized sound-making is purely subjective. You simply have to discard that perception.
I would like a better set of sounds than 'quiet street' and 'noisy street' ... but that's going to take even more research than I've done now, and I'd like to start to get a handle on what kind of sounds I'm looking for. The thing is, most examples that are given bear a certain modernistic similarities - I got better traction by looking up specific things. I'm going to have to spend a bunch of time thinking up a list and then looking up those things and placing them on the decibel scale.
Meanwhile, the system in general should work.
See, figure what the sound level is, then ask yourself as DM, is the party close enough to hear that? Then consider this ... can the thief pick out the sound that's relevant apart from other sounds? Not just that there's voices, but what the voices are actually saying.
That ought to be a gradient as well. However, while I've been piecing out the above, I'm stumped for how to improve the thief's actual ability to decipher the noise. I don't like the idea of a straight percentage roll ... which sucks. Perhaps an ability to nail down sounds that fall into a certain range, and then that range expanding. Between 57 and 62 at 1st level, then between 56 and 63 at second, etc. Note that the range system above allows a very precise measurement of where the sound overlaps that range, and the difference in a few points could make a big difference.
But I'm just spitballin' here at the end. I want to think about it more, try out the base template, and see if I don't get an epiphany.