I wouldn't get too caught up on the L1 on/off issue; the fact remains that when L1 is enabled properly, ISA sound in DOS does not function. I have also witnessed this problem on the Lucky Star LS-486E series of motherboards, based on the SiS 496/497. However, for my case, if I the PCI bus set equal to the FSB, DOS (and Windows) sound works fine for me with my OPL3 card. The issue for me comes when setting PCI = 1/2*FSB, for which case sound FX in DOS will hang the system, although the wavetable header will still function with midi playback. Unlike the situation that pshipkov noticed, I am unable to get Windows sound working when PCI = 1/2*FSB.
I'm using an Audican32 plus, which should be a yamaha OPL3 card, I think the OPL3SAx. pshipkov is using the AWE64Gold. I also tested my DOS sound woes using an ESS ES1868 and confirmed that there is no DOS sound when PCI = 1/2*FSB.
I think the main takeaway from pshipkov's [potentially incorrect method] scheme to disable L1 was that there are some conditions that exist, however detrimental, which will make DOS sound work. Maybe there are other less detrimental means of getting the sound working. I think this is what the OP is looking for.
By the way, the reason I'm not wanting to set PCI = FSB is because I'm working with FSB's of 60-66 MHz. So far, ISA sound, PCI SCSI, and PCI Ultra DMA do not like 1/2*FSB, however my Intel 10/100 PCI NIC seemed to function OK under this condition.
Plan your life wisely, you'll be dead before you know it.