When did Soul Music occur, relative to the other stories? Discworld seemed to be more technologically advanced and closer to Victorian times than in the other books but the main character appeared in other stories and interacted with guys from the present.
Discworld chronology is a can of worms best left unopened. Thief Of Time explains why.
Soul Music didn't happen. Read the end of the book again - Death rewrites all of the books and makes sure that Imp goes to Pseudopolis instead of Ankh-Morpork, and with the exception of himself and Susan, it will only be vaguely remembered by everyone as if in a dream. Why Vetinari can casually refer to '...that Music With Rocks In business a few years ago...' in The Truth and be understood by the other characters, however, is a different matter entirely...
More than that, Ridcully remembers in Hogfather having breakfast with Susan in Soul Music, which was a minor event, and which ought to have been easily erased.
Although, of course, if wizards can see things that are really there, why not remember things that— well, had had really happened?
Given that the subplots with Crash's band and Mr. Clete were wrapped up after Death defeated the Music and sent Imp and his friends away to their altered lives, it's unlikely that the novel's entire plot was ever intended to have been Retconned out of history. Rather, the past was edited just enough so the Band With Rocks In had a different lineup and Imp, Glod, Lias and Asphault were doing something else in the interim, much like how Susan had been "sitting her exams" instead of riding around being Death.
Heck, given Pterry's love of dramatic irony, Death probably revised things so Mr. Clete and his Guild thugs got billing on the Band With Rocks In posters. They'd been killed on Dead Man's Curve too, so he may as well keep any alterations to history as simple (and amusingly ironic) as possible.
If you really must try, go by character's ages, rather than apparent time periods. Which puts Soul Music ... somewhere under twenty years after Mort, and a few years before Hogfather, if I'm not mistaken.
It becomes more complicated when you remember that, technically, at least eighteen years have passed between the beginning of Wyrd Sisters and the end of it, since the witches removed Lancre from time. Pyramids can probably take place anywhen, and Small Gods seems to take place in the past (at least as far as the past can exist when the History Monks are concerned.) Mad Lord Snapcase must not have lasted too long on the throne if Vetinari's been the patrician from The Colour of Magic onward. Unless he did. Remember, there are no timeline problems in the Discworld, only alternate pasts.
Pyramids took place in the present (whatever that means on the Disc) - Dr Cruces is already head tutor in that book, so it can't have taken place all that long before Men at Arms. Unfortunately this causes a problem with Small Gods: the same Ephebian philosophers (Xeno and Ibid) who were around in Pyramids are still around, so Small Gods also takes place in the present... the problems with Omnian and Ephebian history are both Lampshaded in Thief of Time.
While this is probably justifying a Plot Hole, I thought that it could be interpreted that the philosophers in Pyramids had the same names as those in Small Gods who were famous. For example, there is a Greek philosopher Isocrates who lived after the more famous Socrates. If they were shown as having the same personalities as the characters in Small Gods, then disregard this statement.
"Seen through her business eyes, History was very strange indeed. The scars stood out. The history of the country of Ephebe was puzzling for example. Either its famous philosophers lived a very long time, or inherited their names, or extra bits had been stitched into history there. The history of Omnia was a mess. Two centuries had been folded into one by the look of it, and it was only because of the mindset of the Omnians, whose religion mixed the past and the future with the present in any case, that it could possibly have passed unnoticed. And what about Koom Valley?..." Seriously. At this point any chronological problems can be explained with "the History Monks Did it."
Alternately, Xeno and Ibid could've been blown into the past by the Great Pyramid's detonation, same as Dios; they just didn't go as far.
Unlikely, since no-one else in the kingdom was sent back, not even Pteppic.
No one that we know of, anyway. But hey, it's Djelibeybi. How many locals, sent back a few hundred years, would even notice anything had changed?
Cliff pays for the guitar with a diamond tooth, which is mentioned to be worth quite a lot. So how come he didn't just use a tooth to pay the entry fee into the Musician's Guild?
Because maybe he didn't want to have to knock out a tooth every time he needed money?
Cliff is not the disk's fastest thinker and by this point he already had to replace Buddy's harp.
Knocking out one of your teeth just so you can get a job isn't really the same as doing so to make amends for accidentally destroying a fellow-musician's beloved instrument. And later on, the Music that was influencing all of them wouldn't have wanted Cliff to pay the entry fee, because it wanted Music With Rocks In to be free for everyone, not just Guild members.
The tooth-knocking-out could be seen as a penance of sort. Cliff was feeling really bad that he'd accidentally destroyed Imp's harp, and getting the guitar was a way of making up for it.