The fadeXXX commands exhibit some inconsistent behavior.
For instance, the fadeRadio command can affect the UI sounds (e.g. button click):

0 fadeRadio 0; //can't hear button clicks, etc. anymore

And directSay is not affected by fadeSpeech (which is what you'd expect based on the name of the command) and instead is affected by fadeRadio:

0 fadeSpeech 0;
_somUnit directSay "SentAmmoLow"; //you can still hear the unit

0 fadeRadio 0;
_somUnit directSay "SentAmmoLow"; //now it works