Is the Sound Blaster 16 device, and more importantly, the OPL emulation, in PCem different to the one in 86Box?
86Box displays the device as a '[ISA] Sound Blaster 16'.
PCem displays it as a 'Sound Blaster 16'.
Both report it in the config as:

1sndcard = sb16

I have discovered that (at least on early versions of Linux), the Yamaha OPL-3 synthesizer of the Sound Blaster 16 in PCem (and QEMU) is either not available nor enabled.

I'm using locally built versions of PCem (17) and the latest of 86Box.

I run a set of automated scripts I've written for the VM guest machine, on PCem, 86Box and QEMU, to install and configure Debian 1.1 (Buzz).
The scripts re-compile the guest kernel with the sound module included, and during the kernel compile, the automated scripts set the kernel config, with the exact same SB16 parameters across all 3 emulators.
In 86Box, the Yamaha OPL-3 synth device is present as a `"Synth device"`, and in PCem and QEMU, it is not.

1$ cat /dev/sndstat
2
3Sound Driver:3.5.2-960330 (Mon Jun 16 11:06:35 JST 2025 root,
4Linux 86box-buzz 2.0.0 #8 Tue Jun 11 09:53:39 PDT 1996 i586)
5Kernel: Linux 86box-buzz 2.0.0 #2 Mon Jun 16 11:11:32 JST 2025 i586
6Config options: 0
7
8Installed drivers: 
9Type 1: OPL-2/OPL-3 FM
10Type 2: SoundBlaster
11Type 6: SoundBlaster 16bit
12Type 7: SB MPU
13
14Card config: 
15SoundBlaster at 0x220 irq 5 drq 1,5
16SB MPU at 0x330 irq 1 drq 0
17OPL-2/OPL-3 FM at 0x388 irq 0 drq 0
18
19Audio devices:
200: SoundBlaster 16 4.5
21
22Synth devices:
230: Yamaha OPL-3
24
25Midi devices:
260: SoundBlaster MPU
27
28Timers:
290: System clock
30
31Mixers:
320: SoundBlaster

The only difference in the output above between 86Box and PCem, is in PCem, the `"Synth devices:"` section is empty.
The line, `"0: Yamaha OPL-3"` is missing.

QEMU is already known to have only support for FM synthesis on the sb16 device/driver it provides.
This can be tested by using playmidi to play MIDI files. On PCem, no playback device can be found.

I tried using both OPL emulators in PCem (DB and Nuked), but the result is the same.

Could the Sound Blaster 16 be affected by the choice of the machine type?
On PCem, I'm using the [Socket 7] Epox P55-VA.
On 86Box, I'm using the [I430VX] Dell Dimension XPS Pxxxa / Mxxxa