Don't know exactly, but I do remember one of the musicians (in a blog) mentioning that Artem brought them a tape for their Quickstep song (stating that it was the only version he could find that was appropriate).
Which indicates either a treasure hunt from the producers, a multiple option from the producers or the pro dancer's own choice (so, basically everything is possible).
Ian Waite said, many series' ago, that the pro dancers went to the producers with a selection of songs they'd like to use and one was then chosen from that.
However, given Ola's recent comments and various other things which have been said, it would appear the producers now give the pros the music and, sometimes, an associated theme.
I kinda thought they had a big list of 1000 pop songs and 10 classical, then another list of the dances, wound up two clockwork mice and set them off. Wherever they landed, that's the combination!
It's the only explanation for that Paso and Tango music really.
Particularly Artem & Natalies Tango music.
