Not a bug, but no where else to ask.
How do I add a second or multiple sounds to the audio blacklist?
How do I reduce the effects for a mod's audio without completely cancelling it with the blacklist?

Depends on what you want to do:
If you want to reduce reverb - Reduce the reflectivity values
If you want to reduce occlusion - Reduce the occlusion values

unfortunately your suggestion only applies to the block behavior in reaction to sounds.
And what I am asking is to reduce sounds from only one mod without outright blacklisting them.