2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/194407A usual causal requirement on a viable theory of matter is that the speed of sound be at most the speed of light. In view of various recent papers querying this limit, the question is revisited here. We point to various issues confronting theories that violate the usual constraint.v2: additional discussion on models that appear to have superluminal signal speeds; version to appear in GRGGeneral Relativity and Quantum CosmologyAstrophysicsHigh Energy Physics - Theorytext