Somewhere halfway through Mission 10 ... you have to activate the teleporter to go to the next level ... but there is another one already active.
If you go through the already active one, the music changes for all that zone. At the end, there's another teleporter that takes you to the same place the "regular" teleporter takes you, and the music changes back to the normal mission soundtrack.
Going through the telepad you activate doesn't change the soundtrack, so it was pretty clear that it is at least a "special" section. Can't remember anything worth noting on that one, except the guards were very heavily armed.