Constant Moderato by Mitsukiyo from Blue Archive Original Soundtrack Vol.1 ~Longing for the memorable days~ [Disc 1]
You may use it with the loading screen mod
Open PAYDAY3\Content\WwiseAudio\Events\Music\Menus\MusicMenu_MainMenu.uexp with hex editor and change these fields.
PAYDAY3\Content\WwiseAudio\Events\Music\Menus\MusicMenu_MainMenu.uexp
Note: double stands for LE 64bit float number, and float for 32bit one