Disappearing Gallop Murmur in 1y cat with heart failure

Species: Cat

Age: 1y

Sex/Neuter: M. Neutered.

Breed: Scottish Fold

Body Weight: ~4kg

History and clinical signs: So a few weeks ago my 1 year old Scottish Fold was super lethargic and tachypneic. We brought him to the Emergency Vet where the vet said he had a Gallop rhythm. We went to see the kitty cardiologist and did an echo he had LVH. They said it was probably HCM but since he's so young it's possible that it's a transient myocarditis and it's possible he returns to normal. We did a troponin to see if the myocardium was being damaged which came back positive (so it may be transient). We've been treating him with LASIX since. He is also an indoor cat, if that helps with any info.

Question: I am a medical student so I've listened to his heart recently and I don't hear a gallop murmur. Obviously I'm used to this in people, not cats, so I'm not 100% sure. Is this a good sign? Can his heart sounds disappear without his LV decreasing in size? The vet cited a study in which it is possible, I can attach if needed.