So yeah, me and a friend of mine randomly found this out.

Basically, first of all you gotta enable Developer Console in your options and bind a key to it.
Then, open a lobby ("Play with Friends" when pressing the Play button), press the key you assigned to the console earlier, and type this in:

X being a random number. Off you go, with a slower or faster soundtrack of the original!

For me, I think that host_timescale 1.5 sounds quite cool, while said friend thinks 1.1 or 2 sound cool. Just our opinions.

So yeah, easy as that. Have fun with it! Also, local games run faster or slower now, so you better go to other servers!