From The TimeSplitters Wiki

If on any level, you fail to accomplish the specified objectives or your health equals zero, the Mission Failure screen appears which allows you to either retry the level or quit the game. It is accompanied by very "in-Your-Face" music that has a very good beat until about mid-way through the music. Then, what appears to be some sort of monster starts growling while the music volume is lowered. When thoroughly analyzed, it is found out that the noises are actually voices with their speeds incredibly slowed down.

Sublimal messages

Since the voices are incredibly slowed down, when their speed is increased, it is hypothesized that the mysterious "monster growls" will sound more human.

A man claimed that he increased the speed by 300% and heard that it was a male voice saying "Stay right where you are" and "Good Lord!" These claims have yet to be proved by another source.

Reverse play

Some players claim that the voices aren't slowed down; they are actually played backwards. This implies that playing the sounds in reverse would make the sounds sound normal.

Some hyphothesize that when played backwards, the "monster growls" actually are Satanic messages encouraging the player to worship Satan. These claims have yet to be disproven as no one has dared play the music backwards.

Is that too loud?

At the end of the Mission Failure music, a woman asking, "Is that too loud?" is heard. This is likely a mistake by Free Radical though it could also be a clever feature added to the game to scare players (in addition to the mysterious noises (see above).