ToME: the Tales of Maj'Eyal

Would it be much of a fuss to implement, apart from the usual (and awesome) system of music, an additional option to turn on "monster sounds".

To describe how it works, it's best to just test-run DoomRL (doom.chaosforge.org) - over there each type of monster have a set of sounds that trigger on various actions. E.g.:
- super uniques might have a "movement" sound, triggering each time the boss moves, with sound volume depending on its proximity
- various monster types might have different "got damaged" sounds (important if a monster gets damaged out of sight line), its volume depending on the integral of monster numbers over the distance
- some actions (abilities) might have specific sounds attached - e.g. using infusions/runes/teleporting/trap springing/etc.

The general ideas would be:
- volume of the extra sound depending on the proximity of the actor and the amount of actors doing it,
- DIRECTION of the sound (left/right channel) depending on the direction of actor vs the player.

If it sounds fuzzy, just load up DoomRL and check some first few levels. And make sure you have headphones on for the superior sound direction perception.

I can do all that, if you find me a shockbolt-like sound artist to make all those sounds

The solution, then, is to kidnap/summon Kornel of DoomRL fame...or one of his helpers---as that game is currently pretty much the frontrunner of such when it comes down to advanced/nifty sound mechanics!

If we do that, we're going to get some kinda cheesy hidden boss like the one after the Cyberdemon in DoomRL, aren't we?

I kid, I kid

Hmm... imagine Trollmire, with Troll-like murder sounds coming from the entrance to the Bill Cove.

Or an area filled with fog and smoke, reducing vision to 3 squares, with some heavy huffing and puffing monster lurking in the darkness.

Or an eerie screeching sound of an erinye (new enemy, darkness aura 3) noticing the player...