Ahhh...I've been searching for this answer myself for a long time....
Anyway open Audio.dat (181MB) with Goldwave version 4 or 5
(
http://www.goldwave.com ).
The format will not be automatically recognized, so you have to manually select to open the file as Dialogic VOX - ADPCM 4 bit, Mono with a sample Rate of 11025 hz.
Now play it and you can hear all referee calls, stadium announcer, and in-game commentary by Dan Stevens & Peter O'Keefe, unfortunately the sound quality isnt perfect so some hiss noises are audible. My guess is that all the audio files are somehow linked & packed inside the big .DAT files, so I suppose someone with some free time would need to code a utility to unpack/pack these files so they can be properly modified, or perhaps create a plugin for Goldwave/Cakewalk/(insert your favourite audio program here) to enable them to decode & edit these files.
audio.dat (181 MB) - In-game commentary, (9 hr track length)
chat.dat (20 MB) - Same deal as audio.dat but contains stadium music tracks, crowd noises, etc. You'll actually be able to listen to fans & other players shouting at other players to wake up, get their act together, etc..... some are rather funny... (1 hr track length)
teams.dat (575KB) - Same deal, but it only has the announcers voice calling all team names, Home Team, Visiting Team, 49ers, Bears, Cowboys, Southwest Alumni, NFL, Superstars, etc. (1:40 min track length)
players.dat (19.8 MB) Announcers calling player numbers, i,e: Number 73, Number, 16, all the way from Number 99 to Number 0. It is then followed by all player names, Johnson, Smith, Aikman, Owens, Favre, etc... (1 hr track length)
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The following dat files don't appear to have sound info on them, judging by their names I think they could have textures or models, but this is only speculation on my part.
faces.dat
helmetlogos.dat
logos.dat
port.dat
You could try and donwsample the files and see if the game runs, I might give it a try the next few days, as I'm testing some hexediting on that game's BIN files.
I tried this on *.dat files from Echelon's NFL2K2 release, so if you are opening the files from your GD-Rom the sample rates should be higher