I'm probably just stating the obvious, but it is something I'v started to do and want to share it - One way to increase the immersion factor of our simulation: I use to tune (via the internet) live atc online stations and leave the player open in the background. Like this I can listen to FSX own sounds, including FSX-ATC and in the same earphone the live ATC. Some careful tuning of the sound levels makes FSX-ATC almost undestiguishable from real live ATC.
Example: in this moment I am in the final leg of a 10 hour flight La Guardia-Paris with a Connie, approaching the UK, so I'v tuned Shannon Center High Altitude. It sounds great and keeps you company in those long flights, plus increasing that "Immersion factor" one thousand times.
NOTE: for ultimate purgatory, do like me: buy yourself an Icom radio transceiver, tune your local Center, connect it to a second earphone in your other ear...
(yes, I'm a perfectly normal/SANE person....!)