[Arkshine]

Forget to say you need to precache the zombie ressources in plugin_precache() since the zombie is not created at map start. When you will have your plugin which spawn the monster_ entities at map start ( plugin_precache ) ; precaching the ressources will be not needed.

But since it's for test, precache them. Here from the HLSDK to show us the ressources used :

const char *CZombie::pAttackHitSounds[] = 
{
    "zombie/claw_strike1.wav",
    "zombie/claw_strike2.wav",
    "zombie/claw_strike3.wav",
};

const char *CZombie::pAttackMissSounds[] = 
{
    "zombie/claw_miss1.wav",
    "zombie/claw_miss2.wav",
};

const char *CZombie::pAttackSounds[] = 
{
    "zombie/zo_attack1.wav",
    "zombie/zo_attack2.wav",
};

const char *CZombie::pIdleSounds[] = 
{
    "zombie/zo_idle1.wav",
    "zombie/zo_idle2.wav",
    "zombie/zo_idle3.wav",
    "zombie/zo_idle4.wav",
};

const char *CZombie::pAlertSounds[] = 
{
    "zombie/zo_alert10.wav",
    "zombie/zo_alert20.wav",
    "zombie/zo_alert30.wav",
};

const char *CZombie::pPainSounds[] = 
{
    "zombie/zo_pain1.wav",
    "zombie/zo_pain2.wav",
};

void CZombie :: Precache()
{
    int i;

    PRECACHE_MODEL("models/zombie.mdl");

    for ( i = 0; i < ARRAYSIZE( pAttackHitSounds ); i++ )
        PRECACHE_SOUND((char *)pAttackHitSounds[i]);

    for ( i = 0; i < ARRAYSIZE( pAttackMissSounds ); i++ )
        PRECACHE_SOUND((char *)pAttackMissSounds[i]);

    for ( i = 0; i < ARRAYSIZE( pAttackSounds ); i++ )
        PRECACHE_SOUND((char *)pAttackSounds[i]);

    for ( i = 0; i < ARRAYSIZE( pIdleSounds ); i++ )
        PRECACHE_SOUND((char *)pIdleSounds[i]);

    for ( i = 0; i < ARRAYSIZE( pAlertSounds ); i++ )
        PRECACHE_SOUND((char *)pAlertSounds[i]);

    for ( i = 0; i < ARRAYSIZE( pPainSounds ); i++ )
        PRECACHE_SOUND((char *)pPainSounds[i]);
}

Adapt for amxx, it will be easy.