It isn't that hard, you just have to make some .wav files that match the heavy current ones and place them at
SteamApps/common/Team Fortress 2/tf/custom/sound/weapons (create those folders if you don't have them, make sure to use these exact names)
You have to name them:
minigun_shoot.wav (normal shooting sound, should loop)
minigun_shoot_crit.wav (same but for crits, should loop)
minigun_wind_up.wav (plays in the time between left-clicking and firing)
minigun_wind_down.wav (plays in the time between releasing left click and the being able to move at full speed again)
minigun_spin.wav (for when you hold right-click, should loop)
minigun_empty.wav (for when you are out of bullets, should loop)