Crystallink

I'm noticing this issue where some modpacks load without sound whatsoever. I check the settings on all of them, any they're always at 100%, but I don't hear anything at all from them. It seems to happen most often with pre-1.7.10 modpacks, (like Big Dig), and some that seem to be 1.8 or 1.9 (like Wizardz Conquest). How do I fix this?