I assume the sound card is connected via the PCI bus.
In this case, first type "lspci -D" to determine the path to your sound card, it has the form domain:bus:slot.function at the beginning of the line. (domain is usually 0000)
Then "echo 1 > /sys/class/pci_bus/domain\:bus/rescan".
From the kernel documentation :
Writing a non-zero value to this attribute will
force a rescan of the device's parent bus and all
child buses, and re-discover devices removed earlier
from this part of the device tree.
Depends on CONFIG_HOTPLUG