From this very thread on Steam (via Lee):
Quote: "You can have MANY sounds overlaying each other if you like. Just create a soundzone, change the script to musicinzone.lua and then assign a music WAV file. Then create a second, third, forth, each with its own track and see how they all try to play at once. For the sound0 and sound1 fields, these are used when the entity uses a script that calls PlaySound(e,0) or PlaySound(e,1) respectively. You can also use the LoopSound, StopSound and similar commands too. "
Intel Core i7-4820K CPU @ 3.70GHz, 16GB RAM, NVIDIA GeForce GTX 770