With soundcards, depends on your needs. Generally, modern onboards are 'good enough', though better audio quality and/or better gaming performance cand be achieved through premium plug in cards.
Network cards are a little more complex. Up to and including 100Mb, it makes little difference unless you need specific features rarely implemented on onboard ones.
Once you get to gigabit (and 10Gb), you also need to consider the implementation. Gigabit will easily flatten a PCI bus, which is why you can't buy gigabit PCI (32Mhz/32bit) cards. I've yet to see a desktop motherboard with PCI-X (64Mhz and/or 64bit) slots. Yet the number of onboard NIC implementations where the manufacturer connects a gigabit NIC to the PCI bus is shockingly common. The likes of ASUS and Gigabyte do this all the time. This has a hugely performance hit on the entire system. Professional boards, such as Intel's own, tend to put the NIC on a dedicated link into the MCH (Northbridge in traditional terms).
As we move more and more away from any PCI on motherboards, and go to PCIe, this becomes less of an issue for 1Gb NICs, but will show its face again with the new 10Gb cards.