[stevedroid]

Just a guess, but when a particular map loads it loads only the sound data only for things that will be used in the level. Voice effects and such are probably loaded as needed. However, because weapon sounds are used frequently, they're probably loaded into memory at the beginning of the level and then simply read from memory. Since you're not supposed to be using that gun on that level, or even the entire mission perhaps, the sound was probably not loaded into memory. When the system seeks to memory to find the sound file it obviously won't. I'm guessing that's the cause of the error.

Solution: Don't cheat!