X670E Steel Legend - PCIe 3.0 x16 Slot (PCIE3)

I see no issues here. If anything, you'd be better off putting the soundcard in the PCIe Slot #3 because that slot is connected directly to the CPU, instead of through the chipset.

Technically, ALL PCIe devices are compatible with ALL PCIe implementations, across generations and lane widths.

You can plug in an RTX 4090 into an x1 slot (physically not possible but can be forced using an adapter) and it should work. Conversely, you can plug in a simple x1 device into an x16 slot and it should just work. 15 of the PCIe lanes won't be used, so it's a waste, but that's nothing new. I mean, a lot of people have x8 GPU (RTX-3050, RX-6600*, etc.) plugged into x16 slots, and "wasting" half of the lanes, but nobody bats an eye.