Does your MAD16 have an 82C929A chipset on it? If it does, let me just say I've had problems with the Windows drivers for that card. Any sound card with the 82C929A chip on it is really meant for use in pure DOS or alongside another sound card in Windows. I've had greater success just running the DOS drivers(ONLY the DOS drivers) for my 82C929A-based OPTi card before completely screwing up the audio amp while trying to reverse the Stereo sound from the YMF262(the left and right audio channels are backwards on a lot of OPTi cards with discrete OPL3).

Now, as for the distorted FM, it could be one of two things:

1) If the card uses an 82C929A or 82C930 chipset along with a real YMF262 and the op-amp right after the YAC512 is an LF347, the distortion is caused by the op-amp
2) If the card uses an 82C931 chipset, it replaced true OPL3 by what's known as OPTiFM, OPTi's piece of crap OPL3 clone

The former can be fixed by replacing the LF347 by a TL074, but the latter can't be fixed as the OPTiFM is integrated within the 82C931.

Could you post a picture of your MAD16?

I think it is a matter of what driver is used, but I don't remember details.

IIRC my Yamaha SW20-PC is 82C929A based, the 9x drivers are VxD based... they work but on anything better than a 486 based machine it's a hit and miss...
you may get it working on a p1 machine or you might get echoes, distortion and all kinds of strange sounds...

I've used an 82C929A on a Windows 95 computer which had a 1GHz Pentium III installed with no problems. I did get distortion, but that was due to the op-amp used to amplify the audio out of the YMF262 not being adequate for the job. If your 82C929A has an LF347 op-amp, the sound will come out distorted unless you use the DOS drivers and don't set the mixer settings too high. The YMF262 will not distort at all if your card has a TL074 op-amp.

What did give me problems under Windows is the later 82C930. The OPL3 is fine, but the SoundBlaster Pro digital audio is almost inaudible. Under DOS, however, everything sounds fine, though the OPL3 is so loud compared to the SoundBlaster Pro digital audio that if you were to max out the mixer settings, you'd only hear OPL3 unless you listen REALLY closely for digital audio.

mine's a 928

MCACMIX helps compensate against opl3 distortion a bit.

Back in those days I had this mad 16 pro and I hated it. In pure dos not everything worked; to be more precise - only a handful of games worked. The card used the 'unnatural' IRQ 10 and I had no idea how to change that back then. In windows 95/98 it sucked even more - I couldnt find a direct X compatible driver and sound acceleration had to be disabled in order to get anything from the card at all.

Oh man I hate that one. I get goose skin while even thinking on it. Switching to CMI 8738 and afterwards to Audigy was a revelation I remember. It was like switching from a S3 Trio V+ to 3dfx Voodoo graphics back then.

Did it have an 82C931 chipset by any chance? Those cards suck and seem to default to IRQ 10.

Personally, I only had issues with OPTi sound cards under Windows 95 or 98 as follows:

-X-Wing Floppy Disk Edition: no sound unless I use the AdLib sound option or use another sound card in conjunction with the OPTi card to cause a conflict at address range 388 so as to get the YMF262 to work(affects the 82C929A, 82C930 and 82C931)
-All games running in a Windows DOS box: SoundBlaster Pro digital audio is almost inaudible(affects only the 82C930)

There is one problem I experienced with my 82C929A under DOS and this was the only game which gave me problems: Descent. I could get Stereo OPL3, but I could not get Stereo SoundBlaster Pro digital audio.

OPTi's earlier chips are a hell of a lot better than their later ones, mainly when it comes to OPL3. Earlier OPTi chips can have either authentic OPL3 or discrete YMF262 clones which are almost exact copies of the YMF262. Later chips like the 82C931 and the later PCI-based 82C935(more commonly known as the MachOne EV1935) use OPTiFM, OPTi's OPL3 clone, and sound like crap as a result, though not as bad as any Analog Devices SoundPort sound card(those have the worst hardware clone of the YMF262 I've ever come across). They seem less compatible, too.