Forum:Call of duty Black Ops Zombies Sound Problem

I have a problem In Call of duty Black Ops Zombies mode In my Xbox 360:

Every time i play a zombies game in a downloaded map like Moon,Verruckt, Shi no numa, Der Riese, Ascension

I play normally the firest 20 rounds; But after certain poit of the game the characters stop talking (Making comments) like : !Stay away from my Vodka¡, or the comments they do after buying a perk or a weapon.

Also the power ups don`t sound after you take them ( Like "Max ammo" ), The round change sound doesn`t appear;

Also the musical easter eggs don`t work (Like "Abracadavre" from Elena Siegman)

It desesperates me because i like their comments and the perk-a-cola machine`s music.

I really don`t know why it happens to me, it have never happened to me in kino der totten or Five...

Only in DLC maps....

If someone know why does it happen and how to solve it please tell me

Another question ( It`s about the Big bang theory achievement ) : Is it possible to complete the Richtofen`s Grand Scheme in 2 player split-screen mode? My friend did the Original characters trapped easter egg and the Eclipse easter egg and i wonder if we can do it in split-screen mode