The "interference/buzzing" sound over external speakers is a well known and THOROUGHLY discussed topic on this site. Use the search feature to learn more. Basically, it's a common problem with any cellphone near a powered speaker.
As far as the airplane mode thing, also a THOROUGHLY discussed topic on here. Choose "No" when asked about airplane mode to retain wifi/bt/cellular functions, and wait and see if the product you are docked with buzzes. If it does, next time choose "Yes" or live with the occasional noise. It's a cellphone issue, not an iPhone specific problem. That's why they give you the option.
Kanyon, if your iPhone is near an alarm/radio, that's the problem. Simply moving it further away would also solve the sudden buzzing problem. I'm surprised that simply laying it flat made it stop. Maybe when it's upright in the dock, it "transmits" the interference slightly further?