Sound in primordial caves seemingly keeps going up and never stops, slowly filling up the max amount of sound effects, however this process is slow.
Leftover sound can be isolated by turning off every sound slider except master volume.

This is with every sound slider off except master.

Tested with:
Alex's Caves
Alex's Mobs
Citadel

which ambient sounds exactly?

Can confirm this is happening on my end as well.
Eventually the sounds get capped and any other sounds therafter simply don't get played.
Leaving the area doesn't even seem to do anything, but relogging does.
Happens with the newest release, 1.1.1 as well.

Update!
I've managed to find a temporary workaround without having to relog.
If you go to the sound settings, you can chose to pick a different sound system from the default one (if you have another choice), which in my case was my headphones.
Switching between the two, even without closing the settings menu, it will reset the sound cap and fix the issue temporarily.
However, eventually the cap will be reached again.
This is tested in the Primordial Caves.

I've managed to isolate the potential culprit, which seems to be the ambient sounds produced by the primordial cave biome.
Turning those sounds off in the sound settings seems to stop the buildup of sounds, which previously led to the sound cap being maxed out.

After using the command /playsound, it seems one of the additions in "alexscaves:primordial_caves_ambience_additions" causes the eventual sound overload.

it was

dinosaurs_0.ogg
dinosaurs_1.ogg
dinosaurs_2.ogg
dinosaurs_3.ogg
dinosaurs_4.ogg
dinosaurs_5.ogg
dinosaurs_13.ogg
dinosaurs_14.ogg

looks like these oggs were compressed in the wrong format

Sorry to poke a closed bug report, but will this also fix the sounds for the Abyssal Chasm?
In that biome the sounds eventually reach the cap as well.
Toxic caves seem to be perfectly fine though.
Haven't tested the other caves yet, since I'm currently just enjoying survival, and am in the Chasm as of this message.

It appears Forlorn Hollows, Abyssal Chasm, and Toxic Caves doesn't have this problem, however Magnetic Caves possibly does.