IIRC, emit_sound plays a sound in the map from a specific location (based on the entity). Only people near that entity will hear the sound. When you use the "spk" client command, it plays solely on that person's computer (i.e. it doesn't sound like it's comping from a specific location in the map and nobody else can hear it unless you're sending it to multiple people).
Regarding "sound stacking", I swear it would actually be the other way around. Would probably need to test it to verify. Also, sounds used with emit_sound need to be precached so that they are available in the engine's sound table (according to
the function documentation).
Also, AMX Mod X 1.8.10 is a version number that has never existed. Do you mean version 1.10.0 (the latest dev version)?