For some reason for the miniguns of AH-9 Pawnee was not added it's own sound sets for weapon sound configuration
CfgWeapons → M134_minigun → StandardSound

Although these sets have been successfully added to the configuration, they are just not used…
CfgSoundSets → M134Minigun_Shot_SoundSet
and CfgSoundSets → M134Minigun_Tail_SoundSet

Look at the extra-steps in the config to add them: https://pastebin.com/0wmvfHw4

				soundsetshot[]=
				{
					"M134Minigun_Shot_SoundSet",
					"M134Minigun_Tail_SoundSet"
				};
soundContinuous=0;
soundBurst=0;

Apparently this should have been done by design on development because for now the sound doesn't have any echo and this is strange behavior, compare with what will happen when we return the sound sets back
If I'm right, the design implies this result from the start

https://youtu.be/txzh5KX14TM