save that (between the hypen lines) as "delme.cpp" in your qb64 folder, and then from the command prompt, from your QB64 folder, run:
internal\c\bin\g++ -s delme.cpp -o delme.exe
which will create "delme.exe". Then, still from the command line, run:
delme.exe
from your qb64 folder. It will try to load "sdl_mixer.dll". If it fails, it will give you an error code. If it succeeds, it should give you the full path of the file that it loaded.
If it can't find "sdl_mixer.dll", it will fail with error 0x7e, which is "ERROR_MOD_NOT_FOUND".