it is, i can show the code i used:
PURGEMODE = true
PURGE = PURGE or {} -- If The Purge should be enabled [Default: true]
PURGE.CountdownTimer = 60 * 30 -- Time until The Purge will start [Default: 60 * 60] 60 Minutes
PURGE.EndCountdownTimer = 60 * 20 -- Time until The Purge will end [Default: 60 * 12] 12 minutes
PURGE.PlayPurgeSoundStart = 52 -- Seconds before The Purge starts, when it should play the starting sound [Default: 52] 52 Seconds
PURGE.PlayPurgeSoundEnd = 1 -- Seconds before The Purge ends, when it should play the end sound [Default: 1] 1 second
PURGE.StartSound = "purge/reveille.mp3" -- The Sound that should play before The Purge starts [Default: "purge/thepurge.mp3"]
PURGE.EndSound = "purge/retreat.mp3" -- The Sound that should play when The Purge ends [Default: "purge/thepurge.mp3"]

PURGE.StartSound = "YOURSOUND.MP3" <---- the name of the mp3 file for your custom sound needs to there and it needs to be the exact same.

I put the mp3 files in the sound folder, changed the settings, but there is no noise, do you know why that is?

PURGE.EndCountdownTimer = 60 * 15 -- Time until The Purge will end [Default: 60 * 12] 12 minutes
PURGE.CountdownTimer = 60 * 30 -- Time until The Purge will start [Default: 60 * 60] 60 Minutes
PURGE.StartSound = "YOURSOUND.MP3" -- The Sound that should play before The Purge starts [Default: "purge/thepurge.mp3"]
PURGE.EndSound = "YOURSOUNDFILE.MP3" -- The Sound that should play when The Purge ends [Default: "purge/thepurge.mp3"]

i don't remember exactly how you change the time and sound effects but if you look around inside the addon long enough you'll find out. thats how i found out.

wow thanks :)

Extract the .GMA and edit the lua file.

How do you shorten the timer?