For a visual of this, google "soundproof ceiling detail" and look at the pictures.

Repetitively. it's OBVIOUS that something happened and my bet is exiting tenant, vendors who were in, or this new tenant but your NOT gonna know who or what until you get it scoped and find out what the impaction/build-up is and from visual a guesstimate of how long it's been down there. 

We had a similar situation last year and put a couple visuals (sheet of paper on the wall with picture) we could change (adding another closet in the master by taking space from the adjoining room - blue taped floor (2k+), extra pantry cabinet (800$$+) ended up selling the weekend after we did all the options and the buyer didn’t even want it, I think they just wanted to know it was fixable if they really really wanted to.

Actually in Little Rock, we are putting in granite throughout, shaker cabinets in several rehabs, 12" x 24" tile in bathrooms and some other really awesome visual upgrades.

I would be really curious to see how you're visualizing the design so it doesn't seem like you're living in a jail cell.

Like how you added the pictures to your post for a visual.  

Cash invested: $200,000Attached ADU built in my San Jose property backyard (630 sqft). 15% annual CoC return.3D visualization: https://my.matterport.com/models/hRAxQheq93H?

FSBO’s are a great way to sell your home without having to pay agent fees and deal with lengthy contracts, just remember to have a bottom line price in mind and visualize how you want your home sale to go.

On my day job, I work on- web programming, e.g. integration a payment system, CRM system- data analysis, data visualization, modelling, etcThis is not an ad.

What a great way to visualize all of it.