After an uptime of several hours my pc's audio would become choppy and started crackling whenever I would load a webpage. Listening to music while surfing became very annoying. After running the LatencyMon app I found the main culprit to be the NDIS.SYS driver which is responsible for the on-board Realtek GBe PCIe NIC. The DPC latencies would spike each time network traffic was active.
While googling I found some people pointing to the BFE as a possible culprit. Disabling and re-enabling the base filtering engine service did cause the same stuttering as the network traffic did, but it did not (temporarily) solve the crackle like it did for some others. The only remedy was rebooting the pc.
Using a torrent client seemed to make the effect occur sooner. So I then wondered if Avast's web shield could be the source of the issue (knowing BFE is used for filtering/safety features) and uninstalled Avast to test this. Lo and behold; No more sound crackle!
Long story short: uninstalling Avast solved my sound stutter/crackling with network traffic after a long system up-time.
Until this is fixed I'll use Windows 8.1's built in Defender as my virus scanner and Windows firewall as my firewall so that I'm able to enjoy my music.
Win 8.1 pro N x64
Avast internet security 2014.9.0.2008
Realtek RTL 8168 NIC (on gigabyte ga-p35c-ds3r)
p.s.: Before you ask. Yes all my drivers were up to date. I cleaned all drivers residues and tried the latest realtek drivers as well as window's own drivers (device manager: remove hardware -> then add new hardware driver download dialogue). Neither solved the issue. EDIT: Removing the firewall from avast solved the issue for now. Consider this option if you are faced with the same issue and nothing else helps!