I'm liking where this is going. Liteloader and Matmos are forgotten, but the need for lovely sounds won't be!
Though, I do have a few sound suggestions.
Calmer underwater sounds while in still or non-ocean/river water would be nice.
Hearing insects or frogs near still water in non-ocean/river biomes (of course, toned down from the swamp sounds)
Lava bubbling and flowing sounds. Oh god, how I loved how matmos did that. Even better, the cave ambience, like distant roaring of a beast or gusts of wind through tunnels.
Maybe sounds cutting out or dampening a bit faster when entering buildings or shelters.
A neat one but might be hard is wind blowing through unsealed windows in a structure. Say for example a castle with no windows or a creeper hole, or even busted out windows in a mansion. Would make places a bit drafty.
Last one I got is trees rusting randomly as the wind picks up. Not too often like Matmos, but just enough to feel natural.
Keep up the awesome work!