When you're on the surface, only wind and distant crows can be hard. Once you go into a cave though, the birds suddenly kick into high gear and sound off. I'm assuming the mod is having a hard time figuring out distinguishing between what's the surface, cave, and sky in the Twilight Forest.

Is it in a biome with huge trees or just an ordinary one?

I think it happens in all the Twilight Forest biomes, though I've only thoroughly checked the "Twilight Swamp", "Twilight Clearing", "Twilight Forest", "Mushroom Forest", and "Enchanted Forest" biomes. I'll probably check all the other biomes and get back to you on it.

Can you type in /ambient-debug and send me a screenshot of it (in cave and on the surface). It would help me to figure out the fix.

Oh, my bad, completely forgot I was gonna check the other biomes. Gimme a sec and I'll give ya some screenshots.

Basically, the biome-specific sounds only play underground for most of the biomes, with the exception of the Twilight Highland and Final Plateau ones which work just fine.

Another strange thing I found is that the forest-related biomes (such as the Dark Forest, Deep Mushroom Forest, Mushroom Forest, Firefly Forest, and Twilight Forest biomes) also played the correct sounds if you were high enough, near the canopy of the trees.

Well, I don't think it would annoy me, but I actually wouldn't be affected by a change like that. I'm working on a modpack that uses your engine to add my own ambient sounds so all of your sounds will probably be changed or removed.

I was trying to figure out a way to get the sounds working properly in the Twilight Forest so I could make all the biomes sound unique and distinct from the Overworld biomes. For example, the fire swamp would sound much more creepy and volcanic than the normal swamp.

Although, I'm probably just going to end up taking your approach with "suspense.end" and create "suspense.twilight_forest" which'll just play everywhere.

Are you familiar with the engine.json (twilight forest dimension) file? There is no tutorial about the new version, but I can help you (if you want to).

Furthermore you can also release your modified version, I would love to link some addons on the curseforge page. Just let me know what you think about it. I will close this issue for now, because it appears like everything is solved (if I'm not mistaken).

Is the suspense.end sound annoying you? This one is actually not bound to caves but to this dimension in general (will always play). I thought it would help the atmosphere but it might also be a little bit too much.