If the glue library is available then *lispbuilder-sdl-audio* is added to *features*. If *lispbuilder-sdl-audio* is not available then you have to use lispbuilder-sdl-mixer for sound. Right now the sdl-glue library has been built for Windows only. I still have to create a makefile for linux.
The point is that your code relies on use-foreign-library signaling an error if the glue library cannot be loaded, but that's not the same thing as it non being there.
On Linux, use-foreign-library returns fine even if the library doesn't exist which means that :lispbuilder-sdl-audio is always pushed to *features* and that causes errors about undefined alien functions when running examples.
-- Stelian Ionescu a.k.a. fe[nl]ix Quidquid latine dictum sit, altum videtur. http://common-lisp.net/project/iolib