It's very strange to me...
I can start up the game and play it without any problems. I can hear "clicks" with the mouse, birds signing, and even some of the classic mercenary one-liners. Still the game seems much more quiet than I recall. For example, there is no music. I'm not sure why, I've turned the sound all the way up, including in-game at the options menu, but still no sound... and the mercs don't seem to talk as much as I recall.
Just for nonsense I'm going to post the sizes of my .slf files located in the data, just in case I had a bad download or something. I'd appreciate it if someone could compare the size with their working game. And, of course, any other help would be appreciated. I'm running 64-bit vista btw.
Music.slf = 92,641 KB
Npc_speech.slf = 151,925 KB
Speech.slf = 193,237 KB
Thanks
PS Should probably add this is for Jagged Alliance 2 Gold v1.12 vanilla...

Music.slf = 101,987 KB
Npc_speech.slf = 177,750 KB
Speech.slf = 226,417 KB
Are you using some kind of on-the-fly / automatic file compression, such as Windows' "compressed folders"? I'm asking because when I manually compress these files with Windows' "compressed (zipped) folder" command, all three files shrink to almost exactly the file sizes you have posted:
Music.ZIP = 92,434 KB
Npc_speech.ZIP = 151,525 KB
Speech.ZIP = 192,696 KB
I don't know if compression alone could actually cause JA2 not to play the music, but it might explain why our file sizes differ. Anyway - the first two messages at http://www.ja-galaxy-forum.com/board/ubbthreads.php?ubb=showflat&Number=131564&page=3 describe a similar problem, and according to http://www.gamespot.com/pc/strategy/jaggedalliance2/show_msgs.php?topic_id=m-1-41190404&pid=197667 , the 1.13 mod introduced a new sound system and might be able to fix audio problems.