I was reading a historical fiction novel (yes, yes, I know) and the author describes Henry VIII as having a “high-pitched, almost squeaky” voice; where could they have gotten that from; would there have been less than flattering contemporary or later descriptions of the way he sounded?
I am not an early modernist at all, but I do not think it would be surprising for a ruler's voice to be described in the sources.
This is occasionally done in for instance in Suetonius' De Vita Caesarum (Lives of the Caesars), a work describing Julius Caesar and the next 11 emperors, which remained popular throughout the Middle Ages and the Renaissance. In the biography of Julius Caesar, chapter 55 (about his oratory) his voice is "said to" have been acuta, sharp or high-pitched. When it comes to the Life of Augustus, by far the most detailed in the work, we learn that the emperor (84.2; from Rolfe's (now public domain) translation) "had an agreeable and rather characteristic enunciation" but that he sometimes used a herald when speaking to the people because of weakness in the throat (a few paragraphs before his recurring illnesses are mentioned). Later Suetonius also writes about the style of oratory and choices of words he would use. In the later biography of Claudius (chap. 30) there is a description of that emperor's physical features, which includes stammering among his "disagreeable traits" to use Rolfe's translation.
I am aware of at least once example of this from the Mediaeval period as well. In Einhard's Vita Karoli Magni, chronicling Charlemagne in a style thought to be inspired by Suetonius, a description of his physical features is given (chap. 22) ending in his voice: it was clear but lesser than would be expected from his appearance (Einhard has just written about how tall and majestic he looked).
I do not know, but I would suspect this was done sometimes in Louis XIII's and Henry VIII's time as well. On the one hand the examples I listed are all from very detailed accounts that were not made for every ruler and period (looking casually I could not find any voice descriptions in the later parts of Suetonius' Lives), but on the other hand far more sources have survived from the early modern period than from Antiquity, so we might find such descriptions in other works like letters. At least Tudor historians did detail the physical features of kings, like the accounts that u/cdesmoulins quotes here. As for them being flattering, it is quite interesting that the biographies of Augustus and Charlemagne, both quite adulatory, both mention some problem with the potentate's voice. The Life of Claudius is on the other hand more negative, and him stammering fits in with that.