You've got to buy the NOS kit upgrade after you win 5 races, then pass 6 pro license tests and you have to win 10 gold stars in order to unlock the BMW M3 "turbo".
That "BMW M3" is just the XRT with modified stats using LFSTweak. Unless you have an incredible knowledge of coding you can not change the physical models of LFS. It is not supported at this time, and is not allowed on these forums.
Sounds in LFS are generated on the fly using information about the engine specs, not using samples. I've not tried it myself but I imagine if you change the engine using LFSTweak, the engine sounds will change accordingly.