This article presents a synthesis environment for physical modeling of valved brass instrument sounds. Synthesis is performed using finite-difference time-domain methods that allow for flexible simulation of time-varying systems. Users have control over the instrument configuration as well as player parameters, such as mouth pressure, lip dynamics, and valve depressions, which can be varied over the duration of a gesture. This article introduces the model used in the environment, the development of code from prototyping in MATLAB and optimization in C, and the incorporation of the executable file in the Sound Loom interface of the Composers Desktop Project. Planned additions to the environment are then discussed. The environment binaries are available to download online along with example sounds and input files.