Seems the GPLv3 is sufficient to ensure that entire PySoy-based games are also free. The key to this is in section 5c of the "last call" draft which specifies the "whole of the work, and all its parts, regardless of how they are packaged" as covered by the terms of the license.
Copyright law does not follow technical rules of how a work is constructed or distributed, it follows human judgment of what a "work" includes. For games this is simple; a game, including all it's parts, includes any media designed for that game. It does not include media arbitrarily loaded by the user, such as generic CD tracks the player of a game selects to be played, which contains no hooks or labels specific to the game or it's engine.
That is, a judge is likely to rule that a song created about alien invasion of length 1 minute 20 seconds, the same length as a cut-screen showing aliens invading, and which is labeled by a convention (file named cutscreen05.ogg) the game uses to determine which music to play, is a part of total work.
However arbitrary media files, such as tracks of a friend's CD, which are not designed specific to the game and which are interactively selected by the user may even be included on the same CD or tarball with the game. Such bundling is called an "aggregate", the license is explicit in permitting this sort of bundling.
I created a game licensing page on our website with more details on this and some ideas for making money using the "gaming as a service" model.