After some testing with the latest 1.151 release on current PW servers, it seems the main cause and fix for the sound problems is simply to make sure the "Disable Music" checkbox in the launcher is not enabled (or make sure rgl_config.txt has "disable_music = 0"), as other people on the main warband and NW boards have suggested - just turn the music slider right down in the game option menu if you don't want to hear it. For me, having disable music set means that no sounds are played most of the time, with occasional bursts of random sounds; while leaving the music enabled just means that a few environmental and coin sounds with sf_2d set are much too loud once inside the hard coded cut off distance for network updates (20m), as far as I can tell, though the priorities are probably wrong for various things.
I did adjust the priorities, volumes, and flags of sounds to what seemed sensible, then uploaded a quick
patch in case people want to apply it and see if things sound better.
In case people skimmed over this post and missed it: the main fix seems to be ensuring the "Disable Music" option in the warband launcher is not set.