Any Xbox 360 owners here?

About a month ago I purchased an Xbox 360. I tested out a few games like Halo CE and Halo 2. Both work fine, but there seems to be an audio issue with Halo 2. Weapons sound okay, but other sound effects in the background, as well as character voices sound strange. The only way I could describe it is like talking into an empty cup. It's as if the sound were recorded in an empty fish tank. It's hard to describe. Halo 1 doesn't have this problem, and on YouTube, there are some videos of H2 on 360 that sound normal. I switched HDMI inputs, used different cables, and even changed the sound settings on my TV and Xbox. Nothing changed. I might be looking for a needle in a haystack in this thread, but if there is anyone who has played these games on the Xbox 360, what could be wrong? Is it the system, the game?

PS: my Xbox 360 is the 250 GB 360 Slim.