Soundfonts are loaded using fopen, which is not unicode-aware on Windows. Suggested solution:

In fluidsynth_priv.h, replace
#define FLUID_FOPEN(_f,_m) fopen(_f,_m)

with something like

#if defined(WIN32)
#define FLUID_FOPEN(_f,_m) _wfopen(_f,_m)
#else
#define FLUID_FOPEN(_f,_m) fopen(_f,_m)
#endif

Reported by: erikronstrom

Original Ticket: fluidsynth/tickets/130