Generally motion of a (conservative) mechanical system can be described as a curve which is a critical point of some Lagrangian functional on some manifold. The Lagrangian is frequently defined by the kinetic and potential energy of the system. Sometimes the motion has constraints which can be described by restricting the motion (the curve) to a sub-manifold. However in realistic cases the constraints are not absolute -- the motion is not in the sub-manifold, but only near it. In these cases one can restrict by adding a potential function which has the sub-manifold as a strict minimum. Then the motion oscillates about the sub-manifold.
The above scheme can be applied to motion of slightly compressible fluids and also to fluids with free boundary and surface tension. In the first case the oscillations will correspond to the sound made by the fluid.