It's the theme of the video "Meet the Sniper" made by Valve.

Meet The Sniper

AUT:AU Dark determination Jhonny (awakening)

Here is the original link to the song by Calio for Jimmy Here

https://www.youtube.com/watch?v=6SGSvGtBGoI

Calio - Monkey Business (JimmyHere Monkey Business Collection)

Jetstream Sam theme (no music)

Jetstream Sam theme (only music)

I NOT created this remix, i just upload from YouTube 
original - https://youtu.be/JfjVIQpJPxA

Saul Goodman 3D but its an Epic Orchestral Version

TF2 Butsexs Spy

Saul Goodman drip (Drippin Jimmy)

source

https://www.youtube.com/watch?v=AvcDRxnBm-Q

(with gun) Engineer Sings About Valve Not Making A Major Update

https://youtu.be/hdcD5BuzGeY 
original video,
"scout go, your just a man from Boston" - Jane   Sparda  - the soilder hunter 
 this song is already amazing but being sung by scout is better

Jetstream Sam theme (no music)

Tifa