When I have all sliders of my sound turned on full, or normal, everything is fine. However, when I turn the slider for "Hostile Mobs" down a bit, or all the way off, it has the same effect on any sounds slime blocks make.
For example: Having the "Hostile Mobs" volume slider at half turns the volume down for slime block sounds by half. So far it affects placing the blocks, destroying them, and bouncing on them.
Steps to recreate:
1. Have all volume sliders on full.
2. Place slime block.
3. Reduce volume of "Hostile Mobs"
4. Place slime blocks.
5. Notice difference in sound.