I recently had a similar issue. My 6 year-old wireless router got toasted by lightning (even though it was on a surge protector) and I had to get a new one. I starting having buzzing and crackling in my speakers. I keep an old pair of speakers around for troubleshooting and got the same result with them. When I disabled the router, it stopped. I then tried switching from my Creative X-Fi sound card to the on-board sound and that works with the wireless router on.
Unless you have a spare set of speakers to help you troubleshoot, it could be a process of elimination.
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