@fanch35 Personally, I think that some kind of breath adjustment is a requirement for the Sylphyo. I cannot imagine not playing without some restriction in the Bell Port.
If you surf around this forum, you will find numerous schemes for controlling breath volume and back pressure. I've posted a set of 3D printer files on Thingiverse. You can see all things related to the Sylphyo (by myself and others) with a search: https://www.thingiverse.com/search?q=sylphyo&type=things&sort=relevant
However, I think your initial issue is still valid: there is a notable difference in volume across the internal sounds ... even with a breath restrictor ...