Best answer by Airgetlam
First, please read this: iOS issue. Pay particular attention to the information in the box at the top. I'd try moving the music to a computer and try playing from that, to see if the issue you're experiencing is indeed what is referenced in the article. It may be, it may not be.
Second, you indicated that your device is adjacent to the router. This isn't a good thing. You should have at least a meter / 3 feet in between your router and the speaker. Both are electrical devices, and can generate interference with each other. As indicated in the wifi interference FAQ, it's still possible to get interference depending on lots of outside sources that aren't in your home. Might be worth reading that, and perhaps trying a channel change on your router, or looking for other potential sources of interference.
They're only moderately priced speakers, but they do depend on a good wifi signal. However, if you wanted to, you could skip the wifi interference issues by connecting all of your speakers to your router with an ethernet cable. Then you'd only need to worry about duplicate IP address issues on your LAN.
But if none of that helps, I would recommend that you submit a system diagnostic within 10 minutes of recreating this issue, and either post the number here, or contact Sonos to discuss it. I tend to suggest the phone folks, they have more tools available, but are only available during business hours. Both the Twitter and Facebook support folks are available 24/7.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.