Hey, I can shed some light on this musical mystery, since I have some knowledge in vintage sound cards and the likes. I know exactly what this is. (WARNING: Boring technical explanation ahead.) While the early Sound Blasters all had the OPL synthesizer, up through the AWE64, later Creative cards (starting with the Live) did not include it. Everything past the AWE64 was based on technology acquired from Ensoniq. Ensoniq's early MIDI-based ISA cards like the Soundscape didn't have an OPL synth at all, but a lot of games only had OPL support and no MIDI yet (except for a select few) so Ensoniq made a sort of compatibility hack for people who didn't have a Sound Blaster to use alongside a Soundscape. What it did was roughly remap OPL waveforms to MIDI instruments. I don't really know the details, but what's for sure is that it was very rough and usually didn't sound that good.
Later, after the Soundscape cards, Ensoniq did a PCI card called the AudioPCI. They said that it was Sound Blaster compatible, which was pretty handy because most PCI cards couldn't do that (a limitation stemming from how games were programmed to access the Sound Blaster in the first place-- through the ISA bus). To my knowledge it was mostly software emulated. While they got the wave audio working, the OPL synth couldn't be done due to the processing limitations of the day. So they jumped back to their hack from the Soundscape and kept on using it. After Creative bought out Ensoniq, they made the Live. By that point Windows 9x had been out for several years meaning Creative didn't really want to bother with DOS compatibility, so they did the bare minimum, making Ensoniq's stuff from the AudioPCI work on the Live-- I suppose just as an act of courtesy.
In layman's terms: The SB Live, which you heard this on, doesn't have the OPL synth that the Lemmings games were programmed for, and tries to be compatible with it by roughly replacing its sounds with MIDI instrument equivalents. So what you heard is its best approximation.
My old Pentium II had an AudioPCI in it, and when I listened to the tape recording from the last page I recognized it immediately. When I played Lemmings and also ONML on there it sounded just like that. If you're looking for a recording, I can get the machine set up again and record it. However it'll be a while, since I'm at college right now and my PII is not. I'll be home for the summer in around 4 weeks, though.