have you tried putting your sound card where the usb was (which you says worked) and then putting the USB in the remaining space effecitly swapping them arround.
Freezeing i think implys a bandwidth problem but its not limited to that there are several options available to you:
1. buy a new motherboard, the one you have is not really surficient i personally dont buy a board withotu 5 pci slots

. AGP motherboard are not that expecive not since its classed at yestodays technology.
2. Buy a USB sound card and run it through yoru USB card in the working slot. USB cound cards can be bought for not much money now.
3. Ensure that there is nothing obstructing your second pci slot, anythign which could be cauing it not to work.
4. Give up and accept you cannot have both.
5. Await for someone else to reply with better solutions.
Im not staff on these boards and my answer is purly guessing off my own experteese. Someone with greater knowlage of this will reply shortly with an answer.
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Qualified in the following fields:
MOS Specialist in most fields.
A+, Networking+, Server+, AVCE ICT (Grade AA)
Cisco CCNA2 - Networking
Sheffield University 3 day wireless networking course - Achieved.
Good knowledge in other fields but not much paperwork to back it up.