Breaking News!(maybe...)
I've completely rewrote this tutorial. Actually, rioforce rewrote it, and I rewrote it here.. uh, let me explain.
You guys all know how I came along and based by research on bartvbl's research into the TUNs, and how after various revisions and researches it was discovered that the TUNs are custom, modified VOX files. And because of this format, we were unable to get good sounding audio in-game.
Well, all that has changed.
No, the custom TUN has not be broken. It is still a mystery. However, a new method for beating normal VOX files in submission making custom music has been developed, and this one, despite having a trivial amount of trial-and-error, actually works, with no inference with sound effects!
You all also know how my brother rioforce is involved in almost all my work, and I mostly in his. So he knows about this stuff too, and what is trying to or is being done. He, too, wants to have custom audio in Racers (it is his favorite LEGO game, after all), and he has taken the research by myself and bart, added his own, and has rewritten the book on LEGO Racers Audio Modding.
Yes, some principles stay the same. You still have to use 22050 Hz audio (via re-sampling if it is not already), you still have use mono audio (VOX does not and never will support stereo), and it still might take two tries to get it perfect, but this tutorial, which still uses Audacity, works every time, with any kind of music.
The topic post has been updated with the new tutorial, a proof video of this new method, and an updated history section (mainly just a past-tense write, but rio's stuff has been added at the bottom).
Let the working LEGO Racers Audio Mods commence!