what's the meaning of "mountingly muffled by the noisy current of his blood" here?
Then one peel stopped. There could be no other explanation for the obvious diminuition in sound.
‘You see!’ said Phrynne.
Gerald sat up straight on the side of the bed.
Almost at once further sections of sound subsided, quickly one after the other, until only a single peal was left, that which had begun the ringing. Then the single peal tapered off into a single bell. The single bell tolled on its own, disjointedly, five or six or seven times. Then it stopped, and there was nothing.
Gerald’s head was a cave of echoes, mountingly muffled by the noisy current of his blood.
‘Oh goodness,’ said Phrynne, turning from the window and stretching her arms above her head. ‘Let’s go somewhere else tomorrow.’ She began to take off her dress.

Q: what's the meaning of "mountingly muffled by the noisy current of his blood" here?

Question

what's the meaning of "mountingly muffled by the noisy current of his blood" here?
Then one peel stopped. There could be no other explanation for the obvious diminuition in sound.
‘You see!’ said Phrynne.
   Gerald sat up straight on the side of the bed.
   Almost at once further sections of sound subsided, quickly one after the other, until only a single peal was left, that which had begun the ringing. Then the single peal tapered off into a single bell. The single bell tolled on its own, disjointedly, five or six or seven times. Then it stopped, and there was nothing.
Gerald’s head was a cave of echoes, mountingly muffled by the noisy current of his blood.
‘Oh goodness,’ said Phrynne, turning from the window and stretching her arms above her head. ‘Let’s go somewhere else tomorrow.’ She began to take off her dress.

Q: what's the meaning of "mountingly muffled by the noisy current of his blood" here?

Answer

*sound of the blood moving...

Answer

mountingly means increasingly. So the sound of the blood in moving through the veins in his head is becoming louder than the echo of the bells.

Answer

This is a metaphor, where the cave represents Gerald's head and the noisy current (these words make me think of a stream flowing through the cave) represents the pulse of his blood.

I understand this to mean that after the bells stop ringing, Gerald can still hear the sound of the bells ringing in his ears. Little by little, the memory of the sound fades and what he hears instead is his own heartbeat pulsing in the veins near his ears. It's unusual to notice the sound your heart is making, so I assume there is some reason for his heart to be beating louder or faster than usual.

That's a lot of English; I hope the answer is helpful to you. Good luck with your studies!