EditVoicepack offers a simple interface to edit the file containing the ATC communication used in Microsoft Flight Simulator 2004: A Century of Flight. It is possible to change the phraseology as well as add airline callsigns, airports, and aircraft types. EditVoicepack already contains thousands of ready made entries; and is able to download new entries from a central server. It also allows you to accelerate the existing voices for a more realistic experience and better handling of traffic dense environments
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You will need .NET passport 1.1 or higher for EditVoicePack to work.