Nether brick and red nether brick double slabs still use the stone breaking sound

Steps to Reproduce:

1. Place two nether brick slabs on top of each other
2. Place two red nether brick slabs on top of each other
3. Break both blocks

Observed Results:
The double slab blocks still use the stone breaking sounds.

Expected Results:
The double slab blocks use the nether brick sound set.

Notes: