You don't run Win95 in DOSBox. It's not supported. It never was supported. That youtuber misleading with this "OPL3 player" of an illegally modified Windows 95 bootleg should be disregarded.

This guide may give a better, less embarrassing idea.

My opinion is similar here. While I got Win95 to run on DOSBox once, it took time and wasn't exactly stable or smooth.
In addition, not all releases of Win95 behaved the same. The program leileilol mentioned is much better for this.
Anyway, if you have got trouble and reach a dead end - just try Win 3.1x or Win 3.0 MME with Sound Blaster Pro 2 or 16 drivers.
They can be used as a simple OPL3 MIDI players, too, since each of it also has got a Media Player included.

(Plus, original 3.1 disk sets pop up comparably often on online auctions or on flea market, car boot sales, etc;
WfW 3.11 is even bundled with MS-DOS 6.2x usually, which no fan of old computers should miss in its collection.)

Some guides for them are available here:

Windows 3.1x DOSBox Guide
Set up a Win311-DOSBox system in less than one minute
Guide: How to run Windows 3.0 Multimedia Edition in DOSBox

Last but not least, SEAL/2 also has got an OPL3 MIDI player built-in.
It's simple and may sound not exactly the same as Windows with Voyetra SuperSAPI synth, but maybe it is good enough for you. 😀
See http://sealsystem.sourceforge.net/ for more information.

Try PCem instead of DoBox.

Emulated Windows 9X is really iffy at best. If you don't want major headaches, find an early 2000's PC with an AGP slot, a cheap TNT2 and you should be good to go.