There are lots of instances of music manipulation. The main reason for this is that it needs to have an audible beat, except for very small periods that are allowed in the rules. In addition, the change of rhythm and intensity means that people will often add additional sounds to give that change of rhythm. An example of this might be the Blues for Klook programme skated by Z/G this year. There is a three-time percussion added for one section, I believe to give a sense of a change of rhythm, which I don't think is in the original track. To me, such additional changes (such as adding a march rhythm, which is another trick used ) are a bit lazy and don't generally work, but the intent is to create a unified piece by using one piece of music, rather than creating a disjointed piece by using two unrelated pieces of music. Another example would be S/B's FD- check it out early in the season, where the change of rhythm and intensity is in terms of additional instrumentation (which might be in the original track), but at GP Helsinki, that section seemed to have been replaced by a whole new piece of music, possibly at the behest of the judges who might have informed that that that section didn't comprise enough of a change of, and that its continual use might result in them having lowered pcs, most likely in terms of composition. There is nothing in the rules that forbids this, and the rules indirectly encourage it by demanding the change of tempo or rhythm, and intensity.
Examples for P/C- there is a finger click added for the SD at Worlds 2017 which was not there for the rest of the season; some of moonlight sonata is not moonlight sonata, and the FD 2015 Worlds had had an audible beat added to it all season, of which I believe Hugo Chouinard (who does a lot of the music manipulation) had a demonstration video of on his website. currently his website is down, but for examples of the sort of thing that is done, I would check that out.