You fix bad acoustics like that, the same way you fix bad access to public transit -- by not moving there. You're ahead of this game, and not the poor sop who missed the noise level because they saw it during the day, or missed the poor transit access because he let the realtor drive him around.
There's simply no way to do it using commonly available materials. Sound is measured in decibels, which is an exponential scale. Every increase of 10db represents 10 TIMES the sound energy. 20db = 100x, 30db = 1000x. That's why your stereo (which gets pretty loud) has a thin little 16ga cord (the smallest legal size), but the big Marshall amp racks at a concert venue have power cables the size of your thumb.
If they have 120db going down in that bar, to get to a sleepable 50db, you're talking about 10,000,000x, and you'd have to dampen 99.99999% of the energy. Not with big springs and vacuum chambers could you do this.