Yeah, sounds familiar. There is really very few free IRQ:s in the <10 range (usable for soundcards), IIRC 1 if LPT and both COM ports are enabled. Disabling the LPT and one (or both) COM ports will free up some IRQs, which are needed if there is more than one sound card installed.
And, btw, the BIOS listing will only list PCI devices IRQs. (Non-PnP) ISA IRQs are H/W based, BIOS knows nothing about them (exception: those devices which are configured in BIOS, which is possible only in modern BIOSes - and usually includes the already mentioned COM and LPT ports).
EDIT: Actually, this does not need to be based on my shady memory; from Wikipedia:
Wikipedia: Interrupt request (PC architecture) wrote:IRQ 0 – system timer (cannot be changed)
IRQ 1 – keyboard controller (cannot be changed)
IRQ 2 – cascaded signals from I […]
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IRQ 0 – system timer (cannot be changed)
IRQ 1 – keyboard controller (cannot be changed)
IRQ 2 – cascaded signals from IRQs 8–15 (any devices configured to use IRQ 2 will actually be using IRQ 9)
IRQ 3 – serial port controller for serial port 2 (shared with serial port 4, if present)
IRQ 4 – serial port controller for serial port 1 (shared with serial port 3, if present)
IRQ 5 – parallel port 2 and 3 or sound card
IRQ 6 – floppy disk controller
IRQ 7 – parallel port 1. It is used for printers or for any parallel port if a printer is not present. It can also be potentially be shared with a secondary sound card with careful management of the port.
IIRC IRQ9 is kind of useful for sound card (on typical old DOS computers without ACPI), but not all (especially older) games might not be compatible with using IRQ9, so it should be avoided if possible. Might misremember, though,