The following Guide is how to Install Custom sound for Halo:MCC Steam workshop
(Note : This is a foolproof step by step guide, even if it looks obvious, there are still people struggle how it works)

Go to Steam workshop
Click on Subscribe on the mod

https://imgur.com/tIvnetK

Scroll down and read the description
Sound file requires downloading it from other sources
Click on the link

https://imgur.com/pp2XoyV

If you choose to use Nexus Mod to download
On the "Standalone FMOD Download", Click on the "Manual Download"

https://imgur.com/nMxwJU5

Extract the downloaded "Standalone FMOD" compress/zip folder

https://imgur.com/SvwDE3D

Cut or Copy the fmod folder

https://imgur.com/xN4Xthz

Go to steam library
Right click on Halo:MCC
Click on Properties

https://imgur.com/VHUOnsu

Go to Local Files
Click on Browse

https://imgur.com/lhfEoCj

Click on Halo reach folder

https://imgur.com/kJpAnKS

Paste the Cut/Copied fmod into the Halo reach folder directly
(Optional, Copy the original fmod first and paste it somewhere, you can get the original fmod back by verifying the Halo:MCC but this is faster.)

https://imgur.com/mQXJDRw

Replace the Original fmod completely

https://imgur.com/ceF2GsT

Launch Halo:MCC with Anti-Cheat disabled.

https://imgur.com/wgJvxY1

Go to Halo:Reach
And you can see the mod's name and start from there.