func_train's default moving sound overflows CHAN_STATIC
If you haven't set up a moving sound for func_train (derivative of CBasePlatTrain) - it would use common\null.wav
for movement.
Because of that, if a map is filled with func_train-s (as on hazard.vmf) - these sounds overflow the CHAN_STATIC soundchannel, therefore NPC sounds cannot play anymore
Edited by UAVXP