I'm pretty sure the Z77 chipset can't run two cards at 16 lanes each, even if your CPU could. I believe you can only run two cards at 8 lanes each. This might depend on the motherboard, but I'm pretty sure that's the case for all Z77 boards.
If you run a card in slots 1 and 3 on the Extreme board, what's the problem with installing the sound card in the PCIE x4 slot? Also, a x1 card is tiny compared to a x16 card, so how much airflow will the sound card really restrict? I doubt it would hinder airflow much.
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