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...do you mean that I need use two different sounds for it?!

no matter how many sounds you use. The more the better. Other games also use different sounds for weapons.
Code:
char s = random(5);
if s == 0 then ent_playsound(...)
if s == 1 then ent_playsound(...)
if s == 2 then ent_playsound(...)
...


or something like that. You could also use SWITCH instead of IF...
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Is that engines bug??

no
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Or thats because A7's sound engine is sucks??

no. But the sound system sucks indeed.
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If you think that I made something wrong in my script, I can post it here.

no, that's normal. If two identical sounds play at nearly the same time, some frequencies of the sound are "earsed".