I have one of those RAP-10s as well.
Noticed a similar issue. My old notes suggest that some game drivers put this card into mono mode and then it kinda sticks to that. But games with the latest Miles Sound System drivers can drive it in stereo. The Miles driver sources mention both mono and stereo operation.

I also just noticed this strange bit in RAP10_Win95.pdf, where it says it needs two DMA channels, one for each channel Left or Right.

DMA Channels The RAP-10 card also uses two DMA channels (Direct Memory Access) for the recording and playback of digital audio. […]
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DMA Channels
The RAP-10 card also uses two DMA channels (Direct Memory Access) for the recording and
playback of digital audio. As with interrupts and addresses, no two devices can use the same
DMAs. The RAP-10 needs a DMA for each channel of audio. This means that in order to
record or play back digital audio in stereo, the RAP-10 will need two DMA channels (left and
right). Other devices in your computer that may use DMA channels are scanners, SCSI
interfaces, tape backup drives, and other sound cards. There are no switch or jumper settings
on the RAP-10 card for DMAs – they are selected entirely with software. The default DMA
settings are 5 and 6, but you will need to select a different pair if one of these is in use by
another device. For monophonic playback, only one DMA channel is required.

That is a great info. Just tested with other games and you are correct.

Descent always plays digital in mono, while GM is sometimes in stereo. Tested then with DOOM and there is great stereo channel separation and sounds great too 😀

One more thing about this card... I didn't use the card much and it seems when I start the computer and go straight to play music, the sound out is very low (almost silent) and distorted - clippings. This goes away after a minute or two. Then the sound volume is normal and without any distortion. This happens with digital and GM sound.

Any ideas what could be wrong? First thing that comes to mind are capacitors. But this card has so many caps...

The caps on these cards are all bad and have begun leaking, same with MPU401 cards. My card is yet to go through the capacitor change but it is in the todo list, I already see corrosion at the pins of several capacitors...

Photo isn't from my card. But I guess it's the same or maybe worse. I can see some stains over this area:

Years ago I bought one of these cards cheap on eBay. Caps were completely dead and leaking. Took them off, gave the card a good bath and recapped with what I had.

The card had nice volume again and sounded absolutely beautiful. Yeah a lot of caps but worth it.

kixs wrote on 2023-07-26, 06:53:

One more thing about this card... I didn't use the card much and it seems when I start the computer and go straight to play musi […]
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One more thing about this card... I didn't use the card much and it seems when I start the computer and go straight to play music, the sound out is very low (almost silent) and distorted - clippings. This goes away after a minute or two. Then the sound volume is normal and without any distortion. This happens with digital and GM sound.

Any ideas what could be wrong? First thing that comes to mind are capacitors. But this card has so many caps...

Those symptoms are identical to a Korg sound module I have. Issue was caps (same surface mount electrolytics). I ended up replacing a handful of them and the problem went away.

You may not need to replace all of them, but if you can pinpoint the problem caps then at least replacing those should help.

It is better to replace all of them, the ones left behind will come to bite you again. These particular capacitors fail in all the equipment they are found in with 100% certanty, they're a known bad product from that timeframe.