[Matt]

The script is 'trying' to read a 'video' that's why. It will only read audio extensions such as .wav .mp3 ect. From my understanding, YouTube is all videos. Maybe this will give you a better understanding (Straight out of the level editor NPC Script commands):

// file operations:
play - filename; plays a sound/music file or whatever can be played with 'Windows Media Player'
play2 - filename,x,y,volume; plays a sound/music at x,y with the specified volume (1=default volume)
playlooped - filename; plays a sound file (*.wav) looped
stopsound - filename; stops playing a sound file (*.wav)