Update your game to 3.20 then you have two variants:
1) if music is in MP3 and located in respective folder of the game, OS is Win98SE or higher and your WMP version is at least 9 then use q2_mp3build. Not recommended for P1-P2;
2) if music is in WAV and located in respective folder of the game, OS is Win95 or higher and your WMP version is lower than 9 then use q2_wavebuild. Music should be titled numerically from 02 to 11.
Both builds are here (
http://www.megaupload.com/?d=95146EA9).
From myself - I don`t think that music will make such boring and monotonous game better, both times when I played it within huge time interval it was rip and absence of music didn`t annoy me as much as the game itself. I even think that such game would win something from absence of music as it emphasizes both unusual warfare routine and Stroggs technological world that give you none of these exciting emotions you expect from any game. And this actually comes from the man who has Q2+addons soundtracks in his TOP-5 of VGM.