Indeed the results of Moncrief [268Jump To The Next Citation Point] are more general than those considered in the standard acoustic gravity papers that followed because they additionally permit a general relativistic curved background. In spite of these impressive results, we consider these papers to be part of the “historical period” for the main reason that such works are philosophically orthogonal to modern developments in analogue gravity. Indeed the main motivation for such works was the study of perfect fluid dynamics in accretion flows around black holes, or in cosmological expansion, and in this context the description via an acoustic effective background was just a tool in order to derive results concerning conservation laws and stability. This is probably why even if temporally reference [268] pre-dates Unruh’s 1981 paper by one year, and  [259, 260, 258] post-date Unruh’s 1981 paper by a few years, there seems to have not been any cross-connection.