Although there are many examples of music being used to reduce tension in moments of crisis and negotiation, two of my favorites are The Congress of Vienna (1814) and The Christmas Truce (1914). Those stories are here:

Beethoven and The Congress of Vienna
The Christmas Truce of WWI

The current application of the impact of music on the brain is becoming more and more widespread. Research into the neuroscience of music is expanding at a rapid pace.

Here are just a few examples: