Use this one instead because you are not checking task on putinserver (+ added some optimizations and shortened the code

):
P.S. Replace mysound.wav with your sound name ....
PHP Code:
#include <amxmodx>
#include <dhudmessage>
#define PLUGIN "HUD MSG for VIP,Admin,Player"
#define VERSION "1.0"
#define AUTHOR "4iTGAME & Dim4ik"
new const welcome_sound[] = "mysound.wav"
public plugin_precache()
{
register_plugin(PLUGIN, VERSION, AUTHOR)
precache_sound(welcome_sound)
}
public client_putinserver(id){
if(task_exists(id))
remove_task(id)
set_task(5.0,"flappybuird",id)
}
public flappybuird(id){
new szName[32],iflags;
get_user_name(id, szName, charsmax(szName))
iflags = get_user_flags(id);
set_dhudmessage(0, 255, 0, 0.28, 0.16, 0, 6.0, 12.0)
show_dhudmessage(id, "Glad to see you %s%s ! Enjoy the game.", (iflags & ADMIN_LEVEL_B) ? "Administrator " : (iflags & ADMIN_LEVEL_H) ? "VIP " : "" ,szName)
client_cmd(id, "spk %s", welcome_sound)
}