I haven't tried updating to v1.74 yet...but I tried converting all the wavs to mp3s. It didn't take that long since most of the files were small, and iTunes was able to convert the lot in a matter of minutes. First I put the mp3s in the relevant directories with the wavs, but no difference. Then I took the wavs out and left just the mp3s and I started to get some (but not all) sound effects.
Now when I capture a city I hear the 'panic' sounds, and if I raze the city I get that sound effect as well. I get the revolution noises and I hear the clicks of various buttons (selecting a unit for example) but I don't get any of the voice responses when you click on a unit, nor do I get the sounds of battle or pillaging.
Two other quirks I noticed...
Firstly I couldn't update the ambient sound folder with mp3s as it caused the game to crash. I could hear the sounds of trees etc all the time on top of the music regardless of the zoom/view level, but when I opened a city screen the music would stop and I could hear city sounds (which previously didn't work). None the less it would cause the game to crash within a few minutes so I had to revert to the wavs.
The second quirk was that the 'resources and improvements' folder under civ iv/assets/sounds/ contained a typo ('resouces') and that typo was hardcoded into the game. I thought I must've accidentally deleted the 'r' but when I corrected the folder name I got a long bunch of error messages in the game about files not being found!
I'm not hopeful that v1.74 will fix the problem, but I'm hoping I might have more luck with snow leopard.