It sounds like what you are looking for is some form of USB over IP, where the USB port on the router would be remotely forwarded to a computer somewhere, so that the computer can make use of the USB device as if it were locally attached (VHCI).
MT did attempt to add support for that, but unfortunately it didn’t work out for v5 or v6: http://forum.mikrotik.com/t/what-is-this-added-support-for-usb-forwarding-over-tcp/56039/1
Hopefully, this will return in v7.
Even once they get it working though, it may not be great for USB sound cards, as those tend to be very sensitive to jitter.
Other USB over IP solutions do exist, such as those at http://www.siig.com/it-products/device-server/usb-over-ip.html ; but I have no idea regarding their performance, reliability, suitability, or quality.
–Eric