This tutorial describes on how to replace the announcer.
Your default announcer is hidden in the game.ini with these options:

[/Script/UnrealTournament.UTPlayerController]
AnnouncerPath=/Game/RestrictedAssets/Blueprints/RewardAnnouncerMale.RewardAnnouncerMale_C

You need to replace this line to set up your own announcer
Make a new blueprint, parenting UTAnnouncer. Lets say you call it MyUT4Announcer in a Mutator folder. Then you have the path for the game.ini: /Game/Mutators/MyUT4Announcer.MyUT4Announcer_C

All you have to do there is, to enter your specific folders, where you will put the sound files. These are the default values for RewardAnnouncerMale. It's just setting up the paths
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Now the work starts. Add your custom sounds to the specific folders, you entered. You don't need the prefix there
The bad thing is, the names have to be the same as the sounds, which UT is using and the list is long

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You may want to copy the default UTsounds into yours and go from there. Renaming and finding the right sounds, may be the hardest work here.
You don't need the prefix for renmaing. For example: Multikill
Once done you can just make a mutator refering your Announcer like this:
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Maybe you can even set the Announcer there by blueprint.
Thats it. Place the game.ini lines, pak the mutator and you are done.