The mod itself would be a toggle: if it's on you'll hear everything in DT, if it's off you'll hear stuff normally.
If I'm selecting DT is because I am about to play with DT, right? So why shouldn't I be able to hear it properly?
Take tournaments for example: you can't make a test run on a DT map if you want to get the rhythm before hand so you use the song preview in DT to do so.
Hmmm, okay I now got your point.