You might have to ensure your drivers are the correct ones for SPDIF, and enable the option. I don't recall the exact version. They work in Windows DOS sessions, but not in pure DOS. Although, IIRC, I've heard some have successfully done this when using the DOS test utility?

In a Win9x command prompt you can get OPL3 output, but you should use Sound Blaster Pro Volume Control TSR to increase the FM volume, because the drivers do not expose volume controls for FM in Windows.

In MS-DOS, you can get output through SPDIF in the 16-bit audio test in setupds.exe, but only if you exit-to-DOS from Windows without a full reboot.