It's not just after 90 minutes that glitches in the audio are introduced. Listen to this audio dump of a sine wave. The sound is perfect before it hits the mixer, but then some very noticeable pops get introduced to the audio, even in the first minute.
http://www.mediafire.com/folder/e54rbvo3...io_Crackle
http://www.mediafire.com/folder/e54rbvo3...io_Crackle
Spoiler: