Why should I? The on-board sound is listed as supported, so it should work. But it does not. Seems that MorphOS team and I have different definitions of "supported". And despite the fact that this issue is reported quite often the only help is "buy a SoundBlaster". No other hints, tricks, debug support (fill in the rest), only this solution: "buy".
And by the way, I have a ATI 9800pro with a big fan blocking one PCI slot and all other slots are equipped with USB 2.0 and a SATA controller. So there is no place for a SB Live.
So why should I buy the card or even MorphOS (despite the fact that I really like my MOS 1.4.5 on my Amiga4000)?
Your explanation makes sense.
If it is listed as supported, it should work.
SB Live cards ARE cheap, but you obviously can not fit one in (without changing your video card cooler).
Hopefully a developer will address your sound issue.
Try bringing it up on MorphZone.
Personally, I'd find a single slot video card, place a low height USB card next to the video card, and install a sound card.