Musk is noted in more medieval recipes for food and perfumes than you can shake a sword at, adding a depth and complexity to foods the same way sandalwood does. A little goes a very long way!
Now modernly, we can get musk from whales (ambergris) also known a whale snot.
Finally ambrette musk (hibiscus ablemoschus).
Period musk is almost impossible to get. Why you may ask? While I was doing a bit of studying for another project (surprise, surprise), I’ve found that the original(ish) musk mentioned in period cooking came from three sources. The Siberian musk deer (M. moschiferus), black musk deer (M. fuscus) Alpine/Himalayan musk deer (M. chrysogaster). The glands of the musk deer were harvested (and small quantities from very small musk deer farms) by removing the scent glands of the deer.
The gland secretions would dry into small black grains that were, and are, prized for the pheromones they exude. The description is that of a rich and earthy smell, almost heavenly. Mostly. Some people found it repulsive, but not so many that the deer aren’t on the verge of extinction now.
The original method for removal was to kill the deer and take the musk glands, without worry about sustainability. This has come back to haunt those who relied on the deer’s scent gland. Few of these animals survive, except in the most remote regions. The price, in period is listed as being twice the weight in gold and modernly 3 to 4 times that price now. (Nabhan, pp. 150-151).
So where does that leave the modern cook? Up a creek for the period musk. There is no way that musk, from the listed deer, is harvested cruelty free or without costing an arm and a leg (throw a kidney in for good measure). There are substitutions. There is a variety of plant based musks with beautiful earthy notes and of course synthetics. If possible go with the plant based. Experiment. Try new things! Always err on the side that more can be added but it’s going to be hard to remove what you’ve already put in!