@.Zar:
The easiest solution would be to use a mod which only changes the sound in tandem with this one.

The second-easiest would be to go to steamapps/common/alien swarm reactive drop/reactive drop/sound/weapons/2d/deagle/, copy deaglefp.wav into a new folder: newfolder/sound/weapons/2d/deagle/,
Repeat this process with the 3d folder and deagle.wav: samenewfolder/sound/weapons/3d/deagle/.
Then place this newfolder in your alien swarm reactive drop/reactivedrop/addons folder, copy into it the addoninfo.txt from the ExampleAddon folder if it exists or look up how to make one.

Any way to disable the custom sound?

it works for me, I just checked

It doesn't work