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[PostalDude]

It is quite simple really,you just replace the wav. soundfile with whatever one you want,just don't replace the horn because the sound won't play right(tried replacing horn with sentinel horn from GTA VC) and it sounded weird.Here are my two soundmods:
http://www.sendspace.com/file/hg969z

[madness]

It's been a while ever since I last played with the sound in DRIVER.
Previously, I remember I had to keep the same Sample Size, Channels, Sample rate and Audio Format.

Audio Format: Cannot just be identified by .WAV what the audio format is encoded in, for example: PCM, ADPCM. Differences here is whats causing the horn not to work.

I also remember a file that listed all the audio files, sample size, channels and sample rates. I believe you can change these according to the specifications of the new audio file. But it's best to leave the file alone and re-encode the file your importing into the game.

I haven't changed the audio in the game for three years now so I can post what I remember. I remember writing up a document for the DriverWiki which back then was going to be a Driver Modding website which is hiding somewhere on one of my Hard Drives while I'll find and post soon during a big cleanup I'm in the middle of.