JACK standalone apps read and write JACK ports which are easily connected to other JACK-compatible sound sources/sinks via qjackctl in Linux or Jack Pilot on a Mac (see §8 and §6 for other examples of using JACK to connect audio streams).
Thus, to feed a soundfile to a standalone FAUST app, run any program that can output a soundfile on a JACK port (such as pd), and connect the programs JACK output ports to the FAUST app input ports using the connect/routing GUI interface of qjackctl or Jack Pilot.
The following convenience scripts are distributed with FAUST:21