Level the floor

You could try something as simple/cheap as hockey pucks.

I had no idea!  (I don't suppose there's a preferred brand of... no no no ignore that.)

When I was growing up playing ice hockey pretty much all the pucks came from Czechoslovakia, so if you can find some of those vintage pucks — since the country doesn’t technically exist anymore — I’d guess those NOS pucks would probably sound best.  Ehem.  I have a similar sloping concrete basement floor issue and have used generic hockey pucks when needed and noticed no sonic degradation FWIW.  That said, since you’re on a concrete slab I’d skew toward getting some granite slabs cut to size at Home Depot, etc. and just do that.  Either way I’m sure you’ll be fine.  If you’re more OCD about it you can see how much it’d cost to level out the basement floor with some added concrete.  I thought about that but my hockey past and the pucks ultimately won out.  Best of luck. 

Ehem.

[exhale]  Well done, well done.

I had no idea that custom-size hunks of granite were cheap, and/or subject to such fine specs.  See, this (and others) are just the info I came in without.

Thanks, all.

A slope of 1 inch every 18 inches?  Across the whole length of the room?  How is your house still standing? 

Make some pads.. Screw four 2x4's together and make a box the size you want. Coat the insides with Crisco. Mix some self leveling concrete Fill to the thickness you prefer. Easy