Hi there, just paste it in your <LFS>/data/engine folder.
Then you just need to load it up in-game. Select the XRT and join the track; press shift+a and click the "L" next to the "dump valve muffler" / "dump valve volume" values. Now just select the file that you saved in the "engine" folder.
Btw (in case you're not aware), you can find more engine sounds in the following threads; these are completely-different sounds for a specific car; not just a blow off valve;
http://www.lfsforum.net/showthread.php?t=18589
http://www.lfsforum.net/showthread.php?t=44432
Same as above, save these in LFS's data/engine folder. To use the sound; click on the "Load" button on on the shift+a menu.
Note: you need to be driving / speccing the car that you need to change the sound. Have fun!
Sorry for off/topic; got too much time on my hands