What sound card did you select in the game? Note that the Opti can not do SB16. You could also try WSS, which allows for 16-bit sound. Not sure if DOOM 2 supports that, though.

I have selected "Sound Blaster" (there are not other possibilities) with the settings of my sound card. The problem is the same on other games like Duke3d, wolf3d ecc.!

Then there's something wrong with the card or how it is set up. I know for sure that the 82C925 works fine with DOOM and Wolf3D, as I have it in one of my builds as well.

What confuses me a bit is this:

I have configured the sound card with Sound Blaster settings, 220 port address, IRQ 5, DMA channel 1 and 5 sound FX to mix.

Is that in DOOM 2? You can not use DMA 5, as - like already written - the card does not support SB16. There's no High DMA in Sound Blaster mode.

What's the correct configuration for this sound card?

Can you post a screenshot of the OPTi sound configuration utility and how your BLASTER variable is set?

My configuration:

In Doom2 the DMA is configured with a value equal to 1. The 5 value is intended for the sounds to remix.