Summary:

Living variants of Coral use stone sounds, when coral sounds have already been implemented in the game and are used for honeycomb blocks. The only coral-related blocks that should be using stone sounds are the dead variants and Coral Blocks.

Cause:

Coral in question does not have the correct sounds assigned in blocks.json. Replacing the "sound" : "stone" lines with "sound" : "coral" for the appropriate blocks.

This does have a limitation: the dead variants of wall-hanging coral fans use the same ID as the regular variants, and are separated via data values, so they would need to be separated to fix this.